Gregorian calendar

Gregorian calendar

Overview
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar
Civil calendar
In any country, the civil calendar is the calendar, or possibly one of several calendars, used within that country for civil, official or administrative purposes. The civil calendar is almost always used for general purposes by people and private organizations....

. It was introduced by Pope Gregory XIII
Pope Gregory XIII
Pope Gregory XIII , born Ugo Boncompagni, was Pope from 1572 to 1585. He is best known for commissioning and being the namesake for the Gregorian calendar, which remains the internationally-accepted civil calendar to this date.-Youth:He was born the son of Cristoforo Boncompagni and wife Angela...

, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull
Papal bull
A Papal bull is a particular type of letters patent or charter issued by a Pope of the Catholic Church. It is named after the bulla that was appended to the end in order to authenticate it....

 known by its opening words Inter gravissimas
Inter gravissimas
Inter gravissimas was a papal bull issued by Pope Gregory XIII on February 24, 1582. The document reformed the Julian calendar and created a new calendar which came to be called the Gregorian calendar, which is used in most countries today.-Description:...

. The reformed calendar was adopted later that year by a handful of countries, with other countries adopting it over the following centuries.
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1582   Pope Gregory XIII announces the Gregorian calendar.

1582   Pope Gregory XIII implements the Gregorian Calendar. In Italy, Poland, Portugal, and Spain, October 4 of this year is followed directly by October 15.

1582   Because of the implementation of the Gregorian calendar this day does not exist in this year in Italy, Poland, Portugal and Spain.

1582   Because of the implementation of the Gregorian calendar, this day is skipped in Italy, Poland, Portugal and Spain.

1582   Because of the implementation of the Gregorian calendar, this day is skipped in Italy, Poland, Portugal and Spain.

1582   Because of the implementation of the Gregorian calendar this day does not exist in this year in Italy, Poland, Portugal and Spain.

1582   Because of the implementation of the Gregorian calendar, this day does not exist in this year in Italy, Poland, Portugal and Spain.

1582   Because of the implementation of the Gregorian calendar, this day does not exist in this year in Italy, Poland, Portugal and Spain.

1582   Because of the implementation of the Gregorian calendar this day does not exist in this year in Italy, Poland, Portugal and Spain.

1582   Because of the implementation of the Gregorian calendar, this day does not exist in this year in Italy, Poland, Portugal and Spain.

 
Encyclopedia
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar
Civil calendar
In any country, the civil calendar is the calendar, or possibly one of several calendars, used within that country for civil, official or administrative purposes. The civil calendar is almost always used for general purposes by people and private organizations....

. It was introduced by Pope Gregory XIII
Pope Gregory XIII
Pope Gregory XIII , born Ugo Boncompagni, was Pope from 1572 to 1585. He is best known for commissioning and being the namesake for the Gregorian calendar, which remains the internationally-accepted civil calendar to this date.-Youth:He was born the son of Cristoforo Boncompagni and wife Angela...

, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull
Papal bull
A Papal bull is a particular type of letters patent or charter issued by a Pope of the Catholic Church. It is named after the bulla that was appended to the end in order to authenticate it....

 known by its opening words Inter gravissimas
Inter gravissimas
Inter gravissimas was a papal bull issued by Pope Gregory XIII on February 24, 1582. The document reformed the Julian calendar and created a new calendar which came to be called the Gregorian calendar, which is used in most countries today.-Description:...

. The reformed calendar was adopted later that year by a handful of countries, with other countries adopting it over the following centuries. The motivation for the Gregorian reform was that the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

 assumes that the time between vernal equinoxes is 365.25 days, when in fact it is presently almost exactly 11 minutes shorter. The error between these values accumulated at the rate of about three days every four centuries, resulting in the equinox occurring on March 11 (an accumulated error of about 10 days) and moving steadily earlier in the Julian calendar at the time of the Gregorian reform. Since the Spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered that this steady movement in the date of the equinox was undesirable.

The Gregorian calendar reform contained two parts, a reform of the Julian calendar as used up to Pope Gregory's time, together with a reform of the lunar cycle used by the Church along with the Julian calendar for calculating dates of Easter
Computus
Computus is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age....

. The reform was a modification of a proposal made by the Calabria
Calabria
Calabria , in antiquity known as Bruttium, is a region in southern Italy, south of Naples, located at the "toe" of the Italian Peninsula. The capital city of Calabria is Catanzaro....

n doctor Aloysius Lilius
Aloysius Lilius
Aloysius Lilius , also variously referred to as Luigi Lilio, Luigi Giglio, or Aluise Baldassar Lilio, was an Italian doctor, astronomer, philosopher and chronologist, and also the "primary author" who provided the proposal that became the basis of the Gregorian Calendar reform of 1582.The crater...

 (or Lilio). Lilius' proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years: this part of the proposal had been suggested before by, among others, Pietro Pitati
Pietro Pitati
For the painter Bonifazio de' Pitati, see Bonifazio Veronese.Pietro Pitati was an Italian astronomer and mathematician...

. Lilio also produced an original and practical scheme for adjusting the epact
Epact
The epact was originally defined as the age of the moon in days on January 1, and occurs primarily in connection with tabular methods for determining the date of Easter...

s of the moon for completing the calculation of Easter dates, solving a long-standing difficulty that had faced proposers of calendar reform.

The Gregorian calendar modified the Julian calendar's regular cycle of leap year
Leap year
A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...

s, years exactly divisible by four, including all centurial years, as follows:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.


In addition to the change in the mean length of the calendar year from 365.25 days (365 days 6 hours) to 365.2425 days (365 days 5 hours 49 minutes 12 seconds), a reduction of 10 minutes 48 seconds per year, the Gregorian calendar reform also dealt with the past accumulated difference between these lengths. Between AD 325 (when the Roman Catholic Church thought the First Council of Nicaea
First Council of Nicaea
The First Council of Nicaea was a council of Christian bishops convened in Nicaea in Bithynia by the Roman Emperor Constantine I in AD 325...

 had fixed the vernal equinox on 21 March), and the time of Pope Gregory's bull in 1582, the vernal equinox had moved backward in the calendar, until it was occurring on about 11 March, 10 days earlier. The Gregorian calendar therefore began by skipping 10 calendar days, to restore March 21 as the date of the vernal equinox.

Because of the Protestant Reformation
Protestant Reformation
The Protestant Reformation was a 16th-century split within Western Christianity initiated by Martin Luther, John Calvin and other early Protestants. The efforts of the self-described "reformers", who objected to the doctrines, rituals and ecclesiastical structure of the Roman Catholic Church, led...

, however, many Western European countries did not initially follow the Gregorian reform, and maintained their old-style systems. Eventually other countries followed the reform for the sake of consistency, but by the time the last adherents of the Julian calendar in Eastern Europe (Russia and Greece) changed to the Gregorian system in the 20th century, they had to drop 13 days from their calendars, due to the additional accumulated difference between the two calendars since 1582.

The Gregorian calendar continued the previous year-numbering system (Anno Domini
Anno Domini
and Before Christ are designations used to label or number years used with the Julian and Gregorian calendars....

), which counts years from the traditional date of the nativity, originally calculated in the 6th century and in use in much of Europe by the High Middle Ages
High Middle Ages
The High Middle Ages was the period of European history around the 11th, 12th, and 13th centuries . The High Middle Ages were preceded by the Early Middle Ages and followed by the Late Middle Ages, which by convention end around 1500....

.
This year-numbering system is the predominant international standard today.

Description


The Gregorian solar calendar is an arithmetical calendar. It counts days as the basic unit of time, grouping them into years of 365 or 366 days; and repeats completely every 146,097 days, which fill 400 years, and which also happens to be 20,871 seven-day week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...

s. Of these 400 years, 303 common years have 365 days and 97 leap years have 366 days. This yields a calendar mean year of exactly 365+97/400 days = 365.2425 days = 365 days, 5 hours, 49 minutes and 12 seconds.

A Gregorian year is divided into twelve month
Month
A month is a unit of time, used with calendars, which was first used and invented in Mesopotamia, as a natural period related to the motion of the Moon; month and Moon are cognates. The traditional concept arose with the cycle of moon phases; such months are synodic months and last approximately...

s:
No. Name Days
1 January
January
January is the first month of the year in the Julian and Gregorian calendars and one of seven months with the length of 31 days. The first day of the month is known as New Year's Day...

 
31
2 February
February
February is the second month of the year in the Julian and Gregorian calendars. It is the shortest month and the only month with fewer than 30 days. The month has 28 days in common years and 29 days in leap years...

 
28 or 29
3 March
March
March is in present time held to be the third month of the year in both the Julian and Gregorian calendars. It is one of the seven months which are 31 days long....

 
31
4 April
April
April is the fourth month of the year in the Julian and Gregorian calendars, and one of four months with a length of 30 days. April was originally the second month of the Roman calendar, before January and February were added by King Numa Pompilius about 700 BC...

 
30
5 May
May
May is the fifth month of the year in the Julian and Gregorian Calendars and one of seven months with the length of 31 days.May is a month of autumn in the Southern Hemisphere and spring in the Northern Hemisphere...

 
31
6 June
June
June is the sixth month of the year in the Julian and Gregorian calendars and one of the four months with a length of 30 days. Ovid provides two etymologies for June's name in his poem concerning the months entitled the Fasti...

 
30
7 July
July
July is the seventh month of the year in the Julian and Gregorian Calendars and one of seven months with the length of 31 days. It is, on average, the warmest month in most of the Northern hemisphere and the coldest month in much of the Southern hemisphere...

 
31
8 August
August
August is the eighth month of the year in the Julian and Gregorian Calendars and one of seven months with a length of 31 days.This month was originally named Sextilis in Latin, because it was the sixth month in the original ten-month Roman calendar under Romulus in 753 BC, when March was the first...

 
31
9 September
September
September is the 9th month of the year in the Julian and Gregorian Calendars and one of four months with a length of 30 days.September in the Southern Hemisphere is the seasonal equivalent of March in the Northern Hemisphere....

 
30
10 October
October
October is the tenth month of the year in the Julian and Gregorian Calendars and one of seven months with a length of 31 days. The eighth month in the old Roman calendar, October retained its name after January and February were inserted into the calendar that had originally been created by the...

 
31
11 November
November
November is the 11th month of the year in the Julian and Gregorian Calendars and one of four months with the length of 30 days. November was the ninth month of the ancient Roman calendar...

 
30
12 December
December
December is the 12th and last month of the year in the Julian and Gregorian Calendars and one of seven months with the length of 31 days.December starts on the same day as September every year and ends on the same day as April every year.-Etymology:...

 
31


Although the month length pattern seems irregular, it can be represented by the arithmetic expression L = 30 + { [ M + floor(M/8) ] MOD 2 }, where L is the month length in days and M is the month number 1 to 12. The expression is valid for all 12 months, but for M = 2 (February) adjust by subtracting 2 and then if it is a leap year add 1.

A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1).

Leap years add a 29th day to February, which normally has 28 days. The essential ongoing differentiating feature of the Gregorian calendar, as distinct from the Julian calendar with a leap day every four years, is that the Gregorian omits 3 leap days every 400 years. This difference would have been more noticeable in modern memory were it not that the year 2000 was a leap year in both the Julian and Gregorian calendar systems.

The intercalary
Intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

 day in a leap year is known as a leap day. Since Roman times 24 February (bissextile) was counted as the leap day, but now 29 February is regarded as the leap day in most countries.

Although the calendar year runs from 1 January to 31 December, sometimes year numbers were based on a different starting point within the calendar. Confusingly, the term "Anno Domini" is not specific on this point, and actually refers to a family of year numbering systems with different starting points for the years. (See the section below for more on this issue.)

Lunar calendar



The Catholic Church
Roman Catholic Church
The Catholic Church, also known as the Roman Catholic Church, is the world's largest Christian church, with over a billion members. Led by the Pope, it defines its mission as spreading the gospel of Jesus Christ, administering the sacraments and exercising charity...

 maintained a tabular lunar calendar, which was primarily to calculate the date of Easter
Easter
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

, and the lunar calendar required reform as well. A perpetual lunar calendar was created, in the sense that 30 different arrangements (lines in the expanded table of epact
Epact
The epact was originally defined as the age of the moon in days on January 1, and occurs primarily in connection with tabular methods for determining the date of Easter...

s) for lunar months were created. One of the 30 arrangements applies to a century (for this purpose, the century begins with a year divisible by 100). When the arrangement to be used for a given century is communicated, anyone in possession of the tables can find the age of the moon on any date, and calculate the date of Easter.

Gregorian reform



The motivation of the Catholic Church in adjusting the calendar was to celebrate Easter at the time it thought the First Council of Nicaea
First Council of Nicaea
The First Council of Nicaea was a council of Christian bishops convened in Nicaea in Bithynia by the Roman Emperor Constantine I in AD 325...

 had agreed upon in 325. Although a canon of the council implies that all churches used the same Easter, they did not. The Church of Alexandria celebrated Easter on the Sunday after the 14th day of the moon (computed using the Metonic cycle
Metonic cycle
In astronomy and calendar studies, the Metonic cycle or Enneadecaeteris is a period of very close to 19 years which is remarkable for being very nearly a common multiple of the solar year and the synodic month...

) that falls on or after the vernal equinox, which they placed on 21 March. However, the Church of Rome still regarded 25 March as the equinox (until 342) and used a different cycle to compute the day of the moon. In the Alexandrian system, since the 14th day of the Easter moon could fall at earliest on 21 March its first day could fall no earlier than 8 March and no later than 5 April. This meant that Easter varied between 22 March and 25 April. In Rome, Easter was not allowed to fall later than 21 April, that being the day of the Parilia
Parilia
thumb|250px|Festa di Pales, o L'estate , a reimagining of the Festival of Pales by [[Joseph-Benoît Suvée]]In ancient Roman religion, the Parilia is an agricultural festival performed annually on April 21, aimed at cleansing both sheep and shepherd. It is carried out in acknowledgment to the Roman...

or birthday of Rome and a pagan festival. The first day of the Easter moon could fall no earlier than 5 March and no later than 2 April. Easter was the Sunday after the 15th day of this moon, whose 14th day was allowed to precede the equinox. Where the two systems produced different dates there was generally a compromise so that both churches were able to celebrate on the same day. By the 10th century all churches (except some on the eastern border of the Byzantine Empire
Byzantine Empire
The Byzantine Empire was the Eastern Roman Empire during the periods of Late Antiquity and the Middle Ages, centred on the capital of Constantinople. Known simply as the Roman Empire or Romania to its inhabitants and neighbours, the Empire was the direct continuation of the Ancient Roman State...

) had adopted the Alexandrian Easter, which still placed the vernal equinox on 21 March, although Bede
Bede
Bede , also referred to as Saint Bede or the Venerable Bede , was a monk at the Northumbrian monastery of Saint Peter at Monkwearmouth, today part of Sunderland, England, and of its companion monastery, Saint Paul's, in modern Jarrow , both in the Kingdom of Northumbria...

 had already noted its drift in 725—it had drifted even further by the 16th century.

Worse, the reckoned Moon that was used to compute Easter
Computus
Computus is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age....

 was fixed to the Julian year by a 19 year cycle
Metonic cycle
In astronomy and calendar studies, the Metonic cycle or Enneadecaeteris is a period of very close to 19 years which is remarkable for being very nearly a common multiple of the solar year and the synodic month...

. However, that approximation built up an error of one day every 310 years, so by the 16th century the lunar calendar was out of phase with the real Moon by four days.

The Council of Trent
Council of Trent
The Council of Trent was the 16th-century Ecumenical Council of the Roman Catholic Church. It is considered to be one of the Church's most important councils. It convened in Trent between December 13, 1545, and December 4, 1563 in twenty-five sessions for three periods...

 approved a plan in 1563 for correcting the calendrical errors, requiring that the date of the vernal equinox be restored to that which it held at the time of the First Council of Nicaea in 325 and that an alteration to the calendar be designed to prevent future drift. This would allow for a more consistent and accurate scheduling of the feast of Easter.

The fix was to come in two stages. First, it was necessary to approximate the correct length of a solar year. The value chosen was 365.2425 days in decimal notation. Although close to the mean tropical year of 365.24219 days, it is even closer to the mean vernal equinox year of 365.2424 days; this fact made the choice of approximation particularly appropriate, as the purpose of creating the calendar was to ensure that the vernal equinox would be near a specific date (21 March). (See Accuracy).

The second stage was to devise a model based on the approximation which would provide an accurate yet simple, rule-based calendar. The formula designed by Aloysius Lilius
Aloysius Lilius
Aloysius Lilius , also variously referred to as Luigi Lilio, Luigi Giglio, or Aluise Baldassar Lilio, was an Italian doctor, astronomer, philosopher and chronologist, and also the "primary author" who provided the proposal that became the basis of the Gregorian Calendar reform of 1582.The crater...

 was ultimately successful. It proposed a 10-day correction to revert the drift since Nicaea, and the imposition of a leap day in only 97 years in 400 rather than in 1 year in 4. To implement the model, it was provided that years divisible by 100 would be leap year
Leap year
A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...

s only if they were divisible by 400 as well
. So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. In this millennium, 2100, 2200, 2300, 2500, 2600, 2700, 2900, and 3000, will not be leap years, but 2400, and 2800 will be. This theory was expanded upon by Christopher Clavius
Christopher Clavius
Christopher Clavius was a German Jesuit mathematician and astronomer who was the main architect of the modern Gregorian calendar...

 in a closely argued, 800 page volume. He would later defend his and Lilius's work against detractors.

The 19-year cycle used for the lunar calendar was also to be corrected by one day every 300 or 400 years (8 times in 2500 years) along with corrections for the years (1700, 1800, 1900, 2100 et cetera) that are no longer leap years. In fact, a new method for computing the date of Easter was introduced.

In 1577 a Compendium was sent to expert mathematicians outside the reform commission for comments. Some of these experts, including Giambattista Benedetti
Giambattista Benedetti
Giambattista Benedetti was an Italian mathematician from Venice who was also interested in physics, mechanics, the construction of sundials, and the science of music.-Science of motion:...

 and Giuseppe Moleto, believed Easter should be computed from the true motions of the sun and moon, rather than using a tabular method, but these recommendations were not adopted.

Gregory dropped 10 days to bring the calendar back into synchronization with the seasons. Lilius originally proposed that the 10-day correction should be implemented by deleting the Julian leap day on each of its ten occurrences during a period of 40 years, thereby providing for a gradual return of the equinox to 21 March. However, Clavius's opinion was that the correction should take place in one move, and it was this advice which prevailed with Gregory. Accordingly, when the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of ten days. The last day of the Julian calendar was Thursday, 4 October 1582 and this was followed by the first day of the Gregorian calendar, Friday, 15 October 1582 (the cycle of weekdays was not affected).

Adoption


Though Gregory's reform was enacted in the most solemn of forms available to the Church, in fact the bull
Papal bull
A Papal bull is a particular type of letters patent or charter issued by a Pope of the Catholic Church. It is named after the bulla that was appended to the end in order to authenticate it....

 had no authority beyond the Catholic Church and the Papal States
Papal States
The Papal State, State of the Church, or Pontifical States were among the major historical states of Italy from roughly the 6th century until the Italian peninsula was unified in 1861 by the Kingdom of Piedmont-Sardinia .The Papal States comprised territories under...

. The changes which he was proposing were changes to the civil calendar
Civil calendar
In any country, the civil calendar is the calendar, or possibly one of several calendars, used within that country for civil, official or administrative purposes. The civil calendar is almost always used for general purposes by people and private organizations....

 over which he had no authority. The changes required adoption by the civil authorities in each country to have legal effect.

The Nicene Council of 325 sought to devise rules whereby all Christians would celebrate Easter on the same day. In fact it took a very long time before Christians achieved that objective (see Easter
Easter
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

 for the issues which arose). However, the bull Inter gravissimas
Inter gravissimas
Inter gravissimas was a papal bull issued by Pope Gregory XIII on February 24, 1582. The document reformed the Julian calendar and created a new calendar which came to be called the Gregorian calendar, which is used in most countries today.-Description:...

became the law of the Catholic Church. It was not recognised, however, by Protestant Churches
Protestantism
Protestantism is one of the three major groupings within Christianity. It is a movement that began in Germany in the early 16th century as a reaction against medieval Roman Catholic doctrines and practices, especially in regards to salvation, justification, and ecclesiology.The doctrines of the...

 nor by Orthodox Churches and others. Consequently, the days on which Easter and related holidays were celebrated by different Christian Churches again diverged.

Adoption in Europe


Only four Catholic countries adopted the new calendar on the date specified by the bull. Other Catholic countries experienced some delay before adopting the reform; and non-Catholic countries, not being subject to the decrees of the Pope, initially rejected or simply ignored the reform altogether, although they all eventually adopted it. Hence, the dates 5 October 1582 to 14 October 1582 (inclusive) are valid dates in many countries, but invalid in others.

Spain, Portugal, the Polish-Lithuanian Commonwealth
Polish-Lithuanian Commonwealth
The Polish–Lithuanian Commonwealth was a dualistic state of Poland and Lithuania ruled by a common monarch. It was the largest and one of the most populous countries of 16th- and 17th‑century Europe with some and a multi-ethnic population of 11 million at its peak in the early 17th century...

, and most of Italy implemented the new calendar on Friday, 15 October 1582, following Julian Thursday, 4 October 1582. The Spanish and Portuguese colonies adopted the calendar later because of the slowness of communication. France adopted the new calendar on Monday, 20 December 1582, following Sunday, 9 December 1582. The Dutch
Dutch Republic
The Dutch Republic — officially known as the Republic of the Seven United Netherlands , the Republic of the United Netherlands, or the Republic of the Seven United Provinces — was a republic in Europe existing from 1581 to 1795, preceding the Batavian Republic and ultimately...

 provinces of Brabant, Zeeland and the Staten-Generaal also adopted it on 25 December of that year, the provinces forming the Southern Netherlands (modern Belgium) on 1 January 1583, and the province of Holland followed suit on 12 January 1583.

Many Protestant countries initially objected to adopting a Catholic invention; some Protestants feared the new calendar was part of a plot to return them to the Catholic fold. In the Czech lands
Czech lands: 1526-1648
Although the Kingdom of Bohemia and the Margravate of Moravia were both under Habsburg rule, they followed different paths of development. Moravians had accepted the hereditary right of the Austrian Habsburgs to rule and thus escaped the intense struggle between native estates and the Habsburg...

, Protestants resisted the calendar imposed by the Habsburg Monarchy
Habsburg Monarchy
The Habsburg Monarchy covered the territories ruled by the junior Austrian branch of the House of Habsburg , and then by the successor House of Habsburg-Lorraine , between 1526 and 1867/1918. The Imperial capital was Vienna, except from 1583 to 1611, when it was moved to Prague...

. In parts of Ireland, Catholic rebels until their defeat in the Nine Years' War
Nine Years' War (Ireland)
The Nine Years' War or Tyrone's Rebellion took place in Ireland from 1594 to 1603. It was fought between the forces of Gaelic Irish chieftains Hugh O'Neill of Tír Eoghain, Hugh Roe O'Donnell of Tír Chonaill and their allies, against English rule in Ireland. The war was fought in all parts of the...

 kept the "new" Easter in defiance of the English-loyal authorities
Kingdom of Ireland
The Kingdom of Ireland refers to the country of Ireland in the period between the proclamation of Henry VIII as King of Ireland by the Crown of Ireland Act 1542 and the Act of Union in 1800. It replaced the Lordship of Ireland, which had been created in 1171...

; later, Catholics practising in secret petitioned the Propaganda Fide for dispensation
Dispensation (Catholic Church)
In the canon law of the Roman Catholic Church, a dispensation is the suspension by competent authority of general rules of law in particular cases...

 from observing the new calendar, as it signalled their disloyalty.

Denmark, which then included Norway and some Protestant states of Germany, adopted the solar portion of the new calendar on Monday, 1 March 1700, following Sunday, 18 February 1700, because of the influence of Ole Rømer, but did not adopt the lunar portion. Instead, they decided to calculate the date of Easter astronomically using the instant of the vernal equinox and the full moon according to Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

's Rudolphine Tables
Rudolphine Tables
The Rudolphine Tables consist of a star catalogue and planetary tables published by Johannes Kepler in 1627 using data from Tycho Brahe's observations.-Previous tables:...

of 1627. They finally adopted the lunar portion of the Gregorian calendar in 1776. The remaining provinces of the Dutch Republic
Dutch Republic
The Dutch Republic — officially known as the Republic of the Seven United Netherlands , the Republic of the United Netherlands, or the Republic of the Seven United Provinces — was a republic in Europe existing from 1581 to 1795, preceding the Batavian Republic and ultimately...

 also adopted the Gregorian calendar in July 1700 (Gelderland), December 1700 (Utrecht and Overijssel) and January 1701 (Friesland and Groningen).

Sweden's relationship with the Gregorian Calendar
Swedish calendar
The Swedish calendar was a calendar in use in Sweden and its possessions from 1 March 1700 until 30 February 1712; it was one day ahead of the Julian calendar and ten days behind the Gregorian calendar...

 was a difficult one. Sweden started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11-day) adjustment gradually, by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. In the meantime, the Swedish calendar would be out of step with both the Julian calendar and the Gregorian calendar for 40 years; also, the difference would not be constant but would change every 4 years. This system had potential for confusion when working out the dates of Swedish events in this 40-year period. To add to the confusion, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were not excluded. The Swedish calendar (according to the transition plan) should now have been 8 days behind the Gregorian, but was still in fact 10 days behind. King Charles XII
Charles XII of Sweden
Charles XII also Carl of Sweden, , Latinized to Carolus Rex, Turkish: Demirbaş Şarl, also known as Charles the Habitué was the King of the Swedish Empire from 1697 to 1718...

 recognised that the gradual change to the new system was not working, and he abandoned it.

However, rather than proceeding directly to the Gregorian calendar, it was decided to revert to the Julian calendar. This was achieved by introducing the unique date 30 February in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the Gregorian calendar in 1753, when Wednesday, 17 February was followed by Thursday, 1 March. Since Finland was under Swedish rule at that time, it did the same. Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...

's annexation to the Russian Empire did not revert this, since autonomy was granted, but government documents in Finland were dated in both the Julian and Gregorian styles. This practice ended when independence was gained in 1917.

Britain
Kingdom of Great Britain
The former Kingdom of Great Britain, sometimes described as the 'United Kingdom of Great Britain', That the Two Kingdoms of Scotland and England, shall upon the 1st May next ensuing the date hereof, and forever after, be United into One Kingdom by the Name of GREAT BRITAIN. was a sovereign...

 and the British Empire
British Empire
The British Empire comprised the dominions, colonies, protectorates, mandates and other territories ruled or administered by the United Kingdom. It originated with the overseas colonies and trading posts established by England in the late 16th and early 17th centuries. At its height, it was the...

 (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752 was followed by Thursday, 14 September 1752. Claims that rioters demanded "Give us our eleven days" grew out of a misinterpretation of a painting by William Hogarth
William Hogarth
William Hogarth was an English painter, printmaker, pictorial satirist, social critic and editorial cartoonist who has been credited with pioneering western sequential art. His work ranged from realistic portraiture to comic strip-like series of pictures called "modern moral subjects"...

. After 1753, the British tax year in Britain continued to operate on the Julian calendar and began on 5 April, which was the "Old Style" new tax year of 25 March. A 12th skipped Julian leap day in 1800 changed its start to 6 April. It was not changed when a 13th Julian leap day was skipped in 1900, so the tax year in the United Kingdom still begins on 6 April.

In Alaska, the change took place when Friday, 6 October 1867 was followed again by Friday, 18 October after the US purchase of Alaska from Russia, which was still on the Julian calendar. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because the International Date Line
International Date Line
The International Date Line is a generally north-south imaginary line on the surface of the Earth, passing through the middle of the Pacific Ocean, that designates the place where each calendar day begins...

 was shifted from Alaska's eastern to western boundary along with the change to the Gregorian calendar.

In Russia the Gregorian calendar was accepted after the October Revolution
October Revolution
The October Revolution , also known as the Great October Socialist Revolution , Red October, the October Uprising or the Bolshevik Revolution, was a political revolution and a part of the Russian Revolution of 1917...

 (so named because it took place in October 1917 in the Julian calendar). On 24 January 1918 the Council of People's Commissars issued a Decree
Soviet Decrees
Decrees were legislative acts of the highest Soviet institutions, primarily of the Council of People's Commissars and of the Supreme Soviet or VTsIK , issued between 1917 and 1924...

 that Wednesday, 31 January 1918 was to be followed by Thursday, 14 February 1918, thus dropping 13 days from the calendar.

The last country of Eastern Orthodox Europe to adopt the Gregorian calendar was Greece on Thursday, 1 March 1923, which followed Wednesday, 15 February 1923 (a change that also dropped 13 days).

Adoption in Eastern Asia


Japan replaced its traditional lunisolar calendar
Japanese calendar
On January 1, 1873, Japan adopted the Gregorian calendar. Before 1873, the Chinese style lunisolar calendar had been in use since 7th century. Japanese eras are still in use.-System:...

 with the Gregorian calendar on , but adopted the numbered months it had used in its traditional calendar in place of European names, and continued to use Gengo
Japanese era name
The Japanese era calendar scheme is a common calendar scheme used in Japan, which identifies a year by the combination of the and the year number within the era...

, reign names, instead of the Common Era
Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...

 or Anno Domini
Anno Domini
and Before Christ are designations used to label or number years used with the Julian and Gregorian calendars....

 system: Meiji
Meiji period
The , also known as the Meiji era, is a Japanese era which extended from September 1868 through July 1912. This period represents the first half of the Empire of Japan.- Meiji Restoration and the emperor :...

1=1868, Taisho
Taisho period
The , or Taishō era, is a period in the history of Japan dating from July 30, 1912 to December 25, 1926, coinciding with the reign of the Taishō Emperor. The health of the new emperor was weak, which prompted the shift in political power from the old oligarchic group of elder statesmen to the Diet...

1=1912, Showa
Showa period
The , or Shōwa era, is the period of Japanese history corresponding to the reign of the Shōwa Emperor, Hirohito, from December 25, 1926 through January 7, 1989.The Shōwa period was longer than the reign of any previous Japanese emperor...

1=1926, Heisei 1=1989, and so on. The "Western calendar" (西暦, seireki) is also widely accepted by civilians and to a lesser extent by government agencies.

Korea adopted the Gregorian calendar on 1 January 1895 with the active participation of Yu Kil-chun
Yu Kil-chun
Yu Kil-chun was a Korean reformist and politician of Korea's late Joseon Dynasty.Born in Seoul, Yu went to Meiji Japan in 1881 to study at Keio University, returning the following year to Korea...

. Although the new calendar continued to number its months, for its years during the Joseon Dynasty
Joseon Dynasty
Joseon , was a Korean state founded by Taejo Yi Seong-gye that lasted for approximately five centuries. It was founded in the aftermath of the overthrow of the Goryeo at what is today the city of Kaesong. Early on, Korea was retitled and the capital was relocated to modern-day Seoul...

, 1895–97, these years were numbered from the founding of that dynasty, regarding year one as 1392. Between 1897 and 1910, and again from 1948 to 1962 Korean era name
Korean era name
Korean era names were used during the period of Silla, Goguryeo, Balhae, Taebong, Goryeo, Joseon, and the Korean Empire. Dangun-giwon, the era name originating from the foundation of Gojoseon is also widely used in Korea as an indication of long civilisation of Korea.-Goguryeo:#Yeongnak Korean era...

s were used for its years. Between 1910 and 1945, when Korea was under Japanese rule
Korea under Japanese rule
Korea was under Japanese rule as part of Japan's 35-year imperialist expansion . Japanese rule ended in 1945 shortly after the Japanese defeat in World War II....

, Japanese era name
Japanese era name
The Japanese era calendar scheme is a common calendar scheme used in Japan, which identifies a year by the combination of the and the year number within the era...

s were used to count the years of the Gregorian calendar used in Korea. From 1945 until 1961 in South Korea, Gregorian calendar years were also counted from the foundation of Gojoseon
Gojoseon
Gojoseon was an ancient Korean kingdom. Go , meaning "ancient," distinguishes it from the later Joseon Dynasty; Joseon, as it is called in contemporaneous writings, is also romanized as Chosŏn....

 in 2333 BCE
Common Era
Common Era ,abbreviated as CE, is an alternative designation for the calendar era originally introduced by Dionysius Exiguus in the 6th century, traditionally identified with Anno Domini .Dates before the year 1 CE are indicated by the usage of BCE, short for Before the Common Era Common Era...

 (regarded as year one), the date of the legendary founding of Korea by Dangun
Dangun
Dangun Wanggeom was the legendary founder of Gojoseon, the first Korean kingdom, around present-day Liaoning, Manchuria, and the Korean Peninsula. He is said to be the "grandson of heaven", and to have founded the kingdom in 2333 BC...

, hence these Dangi (단기) years were 4278 to 4294. This numbering was informally used with the Korean lunar calendar before 1945 but is only occasionally used today. In North Korea
North Korea
The Democratic People’s Republic of Korea , , is a country in East Asia, occupying the northern half of the Korean Peninsula. Its capital and largest city is Pyongyang. The Korean Demilitarized Zone serves as the buffer zone between North Korea and South Korea...

, the Juche calendar has been used since 1997 to number its years, regarding year one as the birth of Kim Il Sung in 1912.

The Republic of China
Republic of China
The Republic of China , commonly known as Taiwan , is a unitary sovereign state located in East Asia. Originally based in mainland China, the Republic of China currently governs the island of Taiwan , which forms over 99% of its current territory, as well as Penghu, Kinmen, Matsu and other minor...

 (ROC) formally adopted the Gregorian calendar at its founding on , but China soon descended into a period of warlordism with different warlords using different calendars. With the unification
Chinese reunification (1928)
Chinese reunification , better known in Chinese history as the Northeast Flag Replacement , is a historical term that refers to Zhang Xueliang's announcement on December 29, 1928 on replacing all banners of the Beiyang Government in Manchuria to the flag of the Nationalist Government, thus...

 of China under the Kuomintang
Kuomintang
The Kuomintang of China , sometimes romanized as Guomindang via the Pinyin transcription system or GMD for short, and translated as the Chinese Nationalist Party is a founding and ruling political party of the Republic of China . Its guiding ideology is the Three Principles of the People, espoused...

 in October 1928, the Nationalist Government decreed that effective the Gregorian calendar would be used. However, China retained the Chinese traditions of numbering the months and a modified Era System, backdating the first year of the ROC to 1912; this system is still in use in Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...

 where the ROC government retains control. Upon its foundation in 1949, the People's Republic of China continued to use the Gregorian calendar with numbered months, but abolished the ROC Era System and adopted Western numbered years.

Adoption by Orthodox Churches


Despite all the civil adoptions, none of the national Orthodox Churches have recognised it for church or religious purposes. Instead, a Revised Julian calendar
Revised Julian calendar
The Revised Julian calendar, also known as the Rectified Julian calendar, or, less formally, New calendar, is a calendar, originated in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the...

 was proposed in May 1923 which dropped 13 days in 1923 and adopted a different leap year rule. There will be no difference between the two calendars until 2800. The Orthodox churches of Constantinople
Ecumenical Patriarchate of Constantinople
The Ecumenical Patriarchate of Constantinople , part of the wider Orthodox Church, is one of the fourteen autocephalous churches within the communion of Orthodox Christianity...

, Alexandria
Greek Orthodox Church of Alexandria
The Greek Orthodox Church of Alexandria, also known as the Patriarchate of Alexandria and All Africa is an autocephalous Greek Orthodox Church within the wider communion of Orthodox Christianity.Officially, it is called the Greek Orthodox Patriarchate of Alexandria to distinguish it from the...

, Antioch, Greece
Church of Greece
The Church of Greece , part of the wider Greek Orthodox Church, is one of the autocephalous churches which make up the communion of Orthodox Christianity...

, Cyprus
Cypriot Orthodox Church
The Church of Cyprus is an autocephalous Greek church within the communion of Orthodox Christianity. It is one of the oldest Eastern Orthodox autocephalous churches, achieving independence from the Patriarchate of Antioch and All the East in 431...

, Romania
Romanian Orthodox Church
The Romanian Orthodox Church is an autocephalous Eastern Orthodox church. It is in full communion with other Eastern Orthodox churches, and is ranked seventh in order of precedence. The Primate of the church has the title of Patriarch...

, and Bulgaria
Bulgarian Orthodox Church
The Bulgarian Orthodox Church - Bulgarian Patriarchate is an autocephalous Eastern Orthodox Church with some 6.5 million members in the Republic of Bulgaria and between 1.5 and 2.0 million members in a number of European countries, the Americas and Australia...

 adopted the Revised Julian calendar, so until 2800 these New calendarists
New calendarists
The New calendarists are those Eastern Orthodox Churches that adopted the Revised Julian calendar, namely the Orthodox churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, Poland, Bulgaria, and most of the Orthodox Church in America...

 would celebrate Christmas on 25 December in the Gregorian calendar, the same day as the Western churches. The Armenian Apostolic Church
Armenian Apostolic Church
The Armenian Apostolic Church is the world's oldest National Church, is part of Oriental Orthodoxy, and is one of the most ancient Christian communities. Armenia was the first country to adopt Christianity as its official religion in 301 AD, in establishing this church...

 adopted the Gregorian calendar in 1923, except in the Armenian Patriarchate of Jerusalem where the old Julian calendar is still in use.

The Orthodox churches of Jerusalem, Russia
Russian Orthodox Church
The Russian Orthodox Church or, alternatively, the Moscow Patriarchate The ROC is often said to be the largest of the Eastern Orthodox churches in the world; including all the autocephalous churches under its umbrella, its adherents number over 150 million worldwide—about half of the 300 million...

, Serbia
Serbian Orthodox Church
The Serbian Orthodox Church is one of the autocephalous Orthodox Christian churches, ranking sixth in order of seniority after Constantinople, Alexandria, Antioch, Jerusalem, and Russia...

, the Republic of Macedonia
Macedonian Orthodox Church
The Macedonian Orthodox Church – Ohrid Archbishopric or just Macedonian Orthodox Church is the body of Christians who are united under the Archbishop of Ohrid and Macedonia, exercising jurisdiction over Macedonian Orthodox Christians in the Republic of Macedonia and in exarchates in the Macedonian...

, Georgia
Georgian Orthodox and Apostolic Church
The Georgian Apostolic Autocephalous Orthodox Church is an autocephalous part of the Eastern Orthodox Church. Since the 4th century AD, Georgian Orthodoxy has been the state religion of Georgia, and it remains the country's largest religious institution....

, Poland
Polish Orthodox Church
The Polish Autocephalous Orthodox Church, commonly known as the Polish Orthodox Church, , is one of the autocephalous Eastern Orthodox Churches in full communion...

 and the Greek Old Calendarists
Greek Old Calendarists
Greek Old Calendarists are groups that separated from the Orthodox Church of Greece or from the Patriarchate of Constantinople, precipitated by disagreement over the abandonment of the traditional Julian Calendar.- History :Up until the early 20th century, the Eastern Orthodox Church used the...

 did not accept the Revised Julian calendar, and continue to celebrate Christmas on 25 December in the Julian calendar, which is 7 January in the Gregorian calendar until 2100. The refusal to accept the Gregorian reforms also has an impact on the date of Easter. This is because the date of Easter is determined with reference to 21 March as the functional equinox, which continues to apply in the Julian calendar, even though the civil calendar in the native countries now use the Gregorian calendar.

All of the other Eastern churches, the Oriental Orthodox
Oriental Orthodoxy
Oriental Orthodoxy is the faith of those Eastern Christian Churches that recognize only three ecumenical councils — the First Council of Nicaea, the First Council of Constantinople and the First Council of Ephesus. They rejected the dogmatic definitions of the Council of Chalcedon...

 churches (Coptic, Ethiopian
Ethiopian Orthodox Tewahedo Church
The Ethiopian Orthodox Tewahedo Church is the predominant Oriental Orthodox Christian church in Ethiopia. The Ethiopian Church was administratively part of the Coptic Orthodox Church until 1959, when it was granted its own Patriarch by Coptic Orthodox Pope of Alexandria and Patriarch of All...

, Eritrean
Eritrean Orthodox Tewahdo Church
The Eritrean Orthodox Tewahedo Church is an Oriental Orthodox church. Its autocephaly was recognised by Pope Shenouda III after Eritrea gained its independence in 1993.-Origins:...

, and Syrian
Syriac Orthodox Church
The Syriac Orthodox Church; is an autocephalous Oriental Orthodox church based in the Eastern Mediterranean, with members spread throughout the world. The Syriac Orthodox Church claims to derive its origin from one of the first Christian communities, established in Antioch by the Apostle St....

) continue to use their own calendars, which usually result in fixed dates being celebrated in accordance with the Julian calendar but the Assyrian Church
Assyrian Church of the East
The Assyrian Church of the East, officially the Holy Apostolic Catholic Assyrian Church of the East ʻIttā Qaddishtā w-Shlikhāitā Qattoliqi d-Madnĕkhā d-Āturāyē), is a Syriac Church historically centered in Mesopotamia. It is one of the churches that claim continuity with the historical...

 uses the Gregorian Calendar as enacted by Mar Dinkha, causing a schism; the Ancient Assyrian Church of the East continues to use the Julian Calendar.

All Eastern churches continue to use the Julian Easter with the sole exception of the Finnish Orthodox Church
Finnish Orthodox Church
The Finnish Orthodox Church is an autonomous Orthodox archdiocese of the Patriarchate of Constantinople. The Church has a legal position as a national church in the country, along with the Evangelical Lutheran Church of Finland....

, which has adopted the Gregorian Easter.

Timeline


The date when each country adopted the Gregorian calendar, or an equivalent, is marked against a horizontal time line. The vertical axis is used for expansion to show separate national names for ease in charting, but otherwise has no significance.

Difference between Gregorian and Julian calendar dates


Since the introduction of the Gregorian calendar, the difference between Gregorian and Julian calendar dates has increased by three days every four centuries:
Gregorian range Julian range Difference
From 15 October 1582
to 10 March 1700
From 5 October 1582
to 28 February 1700
10 days
From 11 March 1700
to 11 March 1800
From 29 February 1700
to 28 February 1800
11 days
From 12 March 1800
to 12 March 1900
From 29 February 1800
to 28 February 1900
12 days
From 13 March 1900
to 13 March 2100
From 29 February 1900
to 28 February 2100
13 days
From 14 March 2100
to 14 March 2200
From 29 February 2100
to 28 February 2200
14 days


A more extensive list is available at Conversion between Julian and Gregorian calendars
Conversion between Julian and Gregorian calendars
The tables below list equivalent dates in the Julian and Gregorian calendars. Years are given in astronomical year numbering.-Conventions:*Within these tables, January 1 is always the first day of the year....

.

This section always places the intercalary day on even though it was always obtained by doubling (the bissextum (twice sixth) or bissextile day) until the late Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

. The Gregorian calendar is proleptic before 1582 (assumed to exist before 1582) while the Julian calendar is proleptic before year AD 1 (because non-quadrennial leap days were used between 45 BC and AD 1).

The following equation gives the number of days (actually, dates) that the Gregorian calendar is ahead of the Julian calendar, called the secular difference between the two calendars. A negative difference means the Julian calendar is ahead of the Gregorian calendar.
where is the secular difference; is the hundreds digits of the year using astronomical year numbering
Astronomical year numbering
Astronomical year numbering is based on AD/CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0, the years before that are designated with negative numbers and the years after that are designated with positive numbers...

, that is, use for BC years; and is the floor function
Floor function
In mathematics and computer science, the floor and ceiling functions map a real number to the largest previous or the smallest following integer, respectively...

 of . The floor function truncates (removes) any decimal fraction of a positive real number , but avoids the ambiguity of truncating a negative number containing a decimal fraction by returning the more negative of its neighboring integer
Integer
The integers are formed by the natural numbers together with the negatives of the non-zero natural numbers .They are known as Positive and Negative Integers respectively...

s .

The calculated difference increases by one in a centurial year (a year ending in '00) at either Julian or Gregorian, whichever is later. For positive differences, Julian is later, whereas for negative differences, Gregorian is later.

Beginning of the year


The year used in dates during the Roman Republic
Roman Republic
The Roman Republic was the period of the ancient Roman civilization where the government operated as a republic. It began with the overthrow of the Roman monarchy, traditionally dated around 508 BC, and its replacement by a government headed by two consuls, elected annually by the citizens and...

 and the Roman Empire
Roman Empire
The Roman Empire was the post-Republican period of the ancient Roman civilization, characterised by an autocratic form of government and large territorial holdings in Europe and around the Mediterranean....

 was the consular year, which began on the day when consuls first entered office—probably 1 May before 222 BC, 15 March from 222 BC and 1 January from 153 BC. The Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

, which began in 45 BC, continued to use 1 January as the first day of the new year. Even though the year used for dates changed, the civil year always displayed its months in the order January through December from the Roman Republican period until the present.

During the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

, under the influence of the Christian Church, many Western European countries moved the start of the year to one of several important Christian festivals—25 December (the Nativity of Jesus
Nativity of Jesus
The Nativity of Jesus, or simply The Nativity, refers to the accounts of the birth of Jesus in two of the Canonical gospels and in various apocryphal texts....

), 25 March (Annunciation
Annunciation
The Annunciation, also referred to as the Annunciation to the Blessed Virgin Mary or Annunciation of the Lord, is the Christian celebration of the announcement by the angel Gabriel to Virgin Mary, that she would conceive and become the mother of Jesus the Son of God. Gabriel told Mary to name her...

), or Easter (France), while the Byzantine Empire
Byzantine Empire
The Byzantine Empire was the Eastern Roman Empire during the periods of Late Antiquity and the Middle Ages, centred on the capital of Constantinople. Known simply as the Roman Empire or Romania to its inhabitants and neighbours, the Empire was the direct continuation of the Ancient Roman State...

 began its year on 1 September and Russia did so on 1 March until 1492 when the year was moved to 1 September.

In common usage, 1 January was regarded as New Year's Day and celebrated as such, but from the 12th century until 1751 the legal year in England began on 25 March (Lady Day
Lady Day
In the western Liturgical year, Lady Day is the traditional name of the Feast of the Annunciation of the Blessed Virgin in some English speaking countries. It is the first of the four traditional English quarter days. The "Lady" was the Virgin Mary. The term derives from Middle English, when some...

). So, for example, the Parliamentary record lists the execution of Charles I
Charles I of England
Charles I was King of England, King of Scotland, and King of Ireland from 27 March 1625 until his execution in 1649. Charles engaged in a struggle for power with the Parliament of England, attempting to obtain royal revenue whilst Parliament sought to curb his Royal prerogative which Charles...

 as occurring in 1648 (as the year did not end until 24 March), although modern histories adjust the start of the year to 1 January and record the execution as occurring in 1649.

Most Western European countries changed the start of the year to 1 January before they adopted the Gregorian calendar. For example, Scotland changed the start of the Scottish New Year to 1 January in 1600 (this means that 1599 was a short year). England, Ireland and the British colonies changed the start of the year to 1 January in 1752 (so 1751 was a short year with only 282 days). Later that year in September the Gregorian calendar was introduced throughout Britain and the British colonies (see the section Adoption). These two reforms were implemented by the Calendar (New Style) Act 1750
Calendar (New Style) Act 1750
The Calendar Act 1750 is an Act of the Parliament of Great Britain...

.

In some countries, an official decree or law specified that the start of the year should be 1 January. For such countries we can identify a specific year when a 1 January-year became the norm. But in other countries the customs varied, and the start of the year moved back and forth as fashion and influence from other countries dictated various customs.
Country Start numbered year
on 1 January
Adoption of
Gregorian Calendar
Denmark Gradual change from
13th to 16th centuries
1700
Venice
Republic of Venice
The Republic of Venice or Venetian Republic was a state originating from the city of Venice in Northeastern Italy. It existed for over a millennium, from the late 7th century until 1797. It was formally known as the Most Serene Republic of Venice and is often referred to as La Serenissima, in...

 
1522 1582
Holy Roman Empire
Holy Roman Empire
The Holy Roman Empire was a realm that existed from 962 to 1806 in Central Europe.It was ruled by the Holy Roman Emperor. Its character changed during the Middle Ages and the Early Modern period, when the power of the emperor gradually weakened in favour of the princes...

 
1544 from 1583
Spain 1556 1582
Portugal 1556 1582
Prussia
Prussia
Prussia was a German kingdom and historic state originating out of the Duchy of Prussia and the Margraviate of Brandenburg. For centuries, the House of Hohenzollern ruled Prussia, successfully expanding its size by way of an unusually well-organized and effective army. Prussia shaped the history...

 
1559 1700
Sweden 1559 1753
France 1564 1582
Southern Netherlands
Southern Netherlands
Southern Netherlands were a part of the Low Countries controlled by Spain , Austria and annexed by France...

 
1576 1582
Lorraine
Lorraine (province)
The Duchy of Upper Lorraine was an historical duchy roughly corresponding with the present-day northeastern Lorraine region of France, including parts of modern Luxembourg and Germany. The main cities were Metz, Verdun, and the historic capital Nancy....

 
1579 1682
Dutch Republic
Dutch Republic
The Dutch Republic — officially known as the Republic of the Seven United Netherlands , the Republic of the United Netherlands, or the Republic of the Seven United Provinces — was a republic in Europe existing from 1581 to 1795, preceding the Batavian Republic and ultimately...

 
1583 from 1582
Scotland 1600
1752
Russia 1700 1918
Tuscany
Tuscany
Tuscany is a region in Italy. It has an area of about 23,000 square kilometres and a population of about 3.75 million inhabitants. The regional capital is Florence ....

 
1721 1750
Britain
Kingdom of Great Britain
The former Kingdom of Great Britain, sometimes described as the 'United Kingdom of Great Britain', That the Two Kingdoms of Scotland and England, shall upon the 1st May next ensuing the date hereof, and forever after, be United into One Kingdom by the Name of GREAT BRITAIN. was a sovereign...

 and
British Empire
British Empire
The British Empire comprised the dominions, colonies, protectorates, mandates and other territories ruled or administered by the United Kingdom. It originated with the overseas colonies and trading posts established by England in the late 16th and early 17th centuries. At its height, it was the...


except Scotland
1752 1752


Neither the papal bull nor its attached canons explicitly fix such a date, though it is implied by two tables of saint
Saint
A saint is a holy person. In various religions, saints are people who are believed to have exceptional holiness.In Christian usage, "saint" refers to any believer who is "in Christ", and in whom Christ dwells, whether in heaven or in earth...

's days, one labelled 1582 which ends on 31 December, and another for any full year that begins on 1 January. It also specifies its epact
Epact
The epact was originally defined as the age of the moon in days on January 1, and occurs primarily in connection with tabular methods for determining the date of Easter...

 relative to 1 January, in contrast with the Julian calendar, which specified it relative to 22 March. These would have been the inevitable result of the above shift in the beginning of the Julian year.

Dual dating


During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1750/51", where the dual year accounts for some countries already beginning their numbered year on 1 January while others were still using some other date. Even before 1582, the year sometimes had to be double dated because of the different beginnings of the year in various countries. Woolley, writing in his biography of John Dee
John Dee (mathematician)
John Dee was an English mathematician, astronomer, astrologer, occultist, navigator, imperialist and consultant to Queen Elizabeth I. He devoted much of his life to the study of alchemy, divination and Hermetic philosophy....

 (1527–1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS.

Old Style and New Style dates


"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which system is used in the British Empire
British Empire
The British Empire comprised the dominions, colonies, protectorates, mandates and other territories ruled or administered by the United Kingdom. It originated with the overseas colonies and trading posts established by England in the late 16th and early 17th centuries. At its height, it was the...

 and other countries that did not immediately change. Because the Calendar Act of 1750 altered the start of the year, and also aligned the British calendar with the Gregorian calendar, there is some confusion as to what these terms mean. They can indicate that the start of the Julian year has been adjusted to start on 1 January (NS) even though contemporary documents use a different start of year (OS); or to indicate that a date conforms to the Julian calendar (OS), formerly in use in many countries, rather than the Gregorian calendar (NS).

Proleptic Gregorian calendar


The Gregorian calendar can, for certain purposes, be extended backwards to dates preceding its official introduction, producing the proleptic Gregorian calendar
Proleptic Gregorian calendar
The proleptic Gregorian calendar is produced by extending the Gregorian calendar backward to dates preceding its official introduction in 1582.-Usage:...

. However, this proleptic calendar should be used with great caution.

For ordinary purposes, the dates of events occurring prior to 15 October 1582 are generally shown as they appeared in the Julian calendar, with the year starting on 1 January, and no conversion to their Gregorian equivalents. The Battle of Agincourt
Battle of Agincourt
The Battle of Agincourt was a major English victory against a numerically superior French army in the Hundred Years' War. The battle occurred on Friday, 25 October 1415 , near modern-day Azincourt, in northern France...

 is universally known to have been fought on 25 October 1415 which is Saint Crispin
Crispin
Saints Crispin and Crispinian are the French Christian patron saints of cobblers, tanners, and leather workers. Born to a noble Roman family in the 3rd century AD, Saints Crispin and Crispinian, twin brothers, fled persecution for their faith, ending up in Soissons, where they preached Christianity...

's Day.

Usually, the mapping of new dates onto old dates with a start of year adjustment works well with little confusion for events which happened before the introduction of the Gregorian Calendar. But for the period between the first introduction of the Gregorian calendar on 15 October 1582 and its introduction in Britain on 14 September 1752, there can be considerable confusion between events in continental western Europe and in British domains in English language histories. Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar. For example the Battle of Blenheim
Battle of Blenheim
The Battle of Blenheim , fought on 13 August 1704, was a major battle of the War of the Spanish Succession. Louis XIV of France sought to knock Emperor Leopold out of the war by seizing Vienna, the Habsburg capital, and gain a favourable peace settlement...

 is always given as 13 August 1704. However confusion occurs when an event affects both. For example William III of England
William III of England
William III & II was a sovereign Prince of Orange of the House of Orange-Nassau by birth. From 1672 he governed as Stadtholder William III of Orange over Holland, Zeeland, Utrecht, Guelders, and Overijssel of the Dutch Republic. From 1689 he reigned as William III over England and Ireland...

 arrived at Brixham
Brixham
Brixham is a small fishing town and civil parish in the county of Devon, in the south-west of England. Brixham is at the southern end of Torbay, across the bay from Torquay, and is a fishing port. Fishing and tourism are its major industries. At the time of the 2001 census it had a population of...

 in England on 5 November 1688 (Julian calendar), after setting sail from the Netherlands on 11 November 1688 (Gregorian calendar).

Shakespeare
William Shakespeare
William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

 and Cervantes
Miguel de Cervantes
Miguel de Cervantes Saavedra was a Spanish novelist, poet, and playwright. His magnum opus, Don Quixote, considered the first modern novel, is a classic of Western literature, and is regarded amongst the best works of fiction ever written...

 apparently died on exactly the same date (23 April 1616), but in fact Cervantes predeceased Shakespeare by ten days in real time (for dating these events, Spain used the Gregorian calendar, but Britain used the Julian calendar). This coincidence, however, historically encouraged UNESCO
UNESCO
The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations...

 to make 23 April the World Book and Copyright Day
World Book and Copyright Day
World Book and Copyright Day is a yearly event on 23 April, organized by UNESCO to promote reading, publishing and copyright...

.

Astronomers avoid this ambiguity by the use of the Julian day number.

For dates before the year 1, unlike the proleptic Gregorian calendar used in the international standard
International standard
International standards are standards developed by international standards organizations. International standards are available for consideration and use, worldwide...

 ISO 8601
ISO 8601
ISO 8601 Data elements and interchange formats – Information interchange – Representation of dates and times is an international standard covering the exchange of date and time-related data. It was issued by the International Organization for Standardization and was first published in 1988...

, the traditional proleptic Gregorian calendar (like the Julian calendar) does not have a year 0
Year zero
"Year zero" does not exist in the widely used Gregorian calendar or in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1...

 and instead uses the ordinal numbers 1, 2, … both for years AD and BC. Thus the traditional time line is 2 BC, 1 BC, AD 1, and AD 2. ISO 8601 uses astronomical year numbering
Astronomical year numbering
Astronomical year numbering is based on AD/CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0, the years before that are designated with negative numbers and the years after that are designated with positive numbers...

 which includes a year 0 and negative numbers before it. Thus the ISO 8601 time line is -0001, 0000, 0001, and 0002.

Months of the year


English speakers sometimes remember the number of days in each month by memorizing a traditional mnemonic
Mnemonic
A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...

 verse:
Thirty days hath September,
April, June, and November.
All the rest have thirty-one,
Excepting February alone,
Which hath twenty-eight days clear,
And twenty-nine in each leap year.


For variations and alternate endings, see Thirty days hath September
Thirty days hath September
Thirty days hath September is a traditional English mnemonic rhyme, of which many variants are commonly used in English-speaking countries to remember the lengths of the months in the Julian and Gregorian calendars....

.
A language-independent alternative used in many countries is to hold up one's two fists with the index knuckle of the left hand against the index knuckle of the right hand. Then, starting with January from the little knuckle of the left hand, count knuckle, space, knuckle, space through the months. A knuckle represents a month of 31 days, and a space represents a short month (a 28- or 29-day February or any 30-day month). The junction between the hands is not counted, so the two index knuckles represent July and August. This method also works by starting the sequence on the right hand's little knuckle, then continuing towards the left. It can also be done using just one hand: after counting the fourth knuckle as July, start again counting the first knuckle as August. A similar mnemonic can be found on a piano keyboard
Musical keyboard
A musical keyboard is the set of adjacent depressible levers or keys on a musical instrument, particularly the piano. Keyboards typically contain keys for playing the twelve notes of the Western musical scale, with a combination of larger, longer keys and smaller, shorter keys that repeats at the...

: starting on the key F for January, moving up the keyboard in semitone
Semitone
A semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music, and it is considered the most dissonant when sounded harmonically....

s, the black notes give the short months, the white notes the long ones.

The Origins of English naming used by the Gregorian calendar:
  • January: Janus
    Janus (mythology)
    In ancient Roman religion and mythology, Janus is the god of beginnings and transitions, thence also of gates, doors, doorways, endings and time. He is usually a two-faced god since he looks to the future and the past...

     (Roman god of gates, doorways, beginnings and endings)
  • February: Februus
    Februus
    In ancient Roman religion, Februus was the god of purification. He was also worshipped under the same name by the Etruscans, as the Etruscan god of purification, but also the underworld...

     (Etruscan
    Etruscan mythology
    The Etruscans were a diachronically continuous population, with a distinct language and culture during the period of earliest European writing, in the Mediterranean Iron Age in the second half of the first millennium BC...

     god of death) Februarius (mensis) (Latin for "month of purification (rituals)" it is said to be a Sabine word, the last month of ancient pre-450 BC Roman calendar
    Roman calendar
    The Roman calendar changed its form several times in the time between the founding of Rome and the fall of the Roman Empire. This article generally discusses the early Roman or pre-Julian calendars...

    ). It is related to fever
    Fever
    Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

    .
  • March: Mars
    Mars (mythology)
    Mars was the Roman god of war and also an agricultural guardian, a combination characteristic of early Rome. He was second in importance only to Jupiter, and he was the most prominent of the military gods worshipped by the Roman legions...

     (Roman god of war)
  • April: "Modern scholars associate the name with an ancient root meaning 'other', i.e the second month of a year beginning in March."
  • May: Maia Maiestas (Roman goddess)
  • June: Juno
    Juno (mythology)
    Juno is an ancient Roman goddess, the protector and special counselor of the state. She is a daughter of Saturn and sister of the chief god Jupiter and the mother of Mars and Vulcan. Juno also looked after the women of Rome. Her Greek equivalent is Hera...

     (Roman goddess, wife of Jupiter)
  • July: Julius Caesar
    Julius Caesar
    Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

     (Roman dictator) (month was formerly named Quintilis, the fifth month of the calendar of Romulus
    Romulus and Remus
    Romulus and Remus are Rome's twin founders in its traditional foundation myth, although the former is sometimes said to be the sole founder...

    )
  • August: Augustus
    Augustus
    Augustus ;23 September 63 BC – 19 August AD 14) is considered the first emperor of the Roman Empire, which he ruled alone from 27 BC until his death in 14 AD.The dates of his rule are contemporary dates; Augustus lived under two calendars, the Roman Republican until 45 BC, and the Julian...

     (first Roman emperor) (month was formerly named Sextilis, the sixth month of Romulus)
  • September: septem (Latin for seven, the seventh month of Romulus)
  • October: octo (Latin for eight, the eighth month of Romulus)
  • November: novem (Latin for nine, the ninth month of Romulus)
  • December: decem (Latin for ten, the tenth month of Romulus)

Week



In conjunction with the system of months there is a system of week
Week
A week is a time unit equal to seven days.The English word week continues an Old English wice, ultimately from a Common Germanic , from a root "turn, move, change"...

s. A physical or electronic calendar provides conversion from a given date to the weekday
Weekday
Weekday may either refer to only a day of the week which is part of the workweek thus not part of the weekend or to any of the days of the week.-Weekday as a day of the workweek:In most countries the days of the workweek are:# Monday# Tuesday# Wednesday...

, and shows multiple dates for a given weekday and month. Calculating the day of the week
Calculating the day of the week
This article details various mathematical algorithms to calculate the day of the week for any particular date in the past or future.A typical application is to calculate the day of the week on which someone was born or some other special event occurred....

 is not very simple, because of the irregularities in the Gregorian system. When the Gregorian calendar was adopted by each country, the weekly cycle continued uninterrupted. For example, in the case of the few countries that adopted the reformed calendar on the date proposed by Gregory XIII for the calendar's adoption, Friday, 15 October 1582, the preceding date was Thursday, 4 October 1582 (Julian calendar).

Distribution of dates by day of the week


Since the 400-year cycle of the Gregorian calendar consists of a whole number of weeks, each cycle has a fixed distribution of weekdays among calendar dates. It then becomes possible that this distribution is not even.

Indeed, because there are 97 leap years in every 400 years in the Gregorian Calendar, there are on average leap years for each starting weekday in each cycle. This already shows that the frequency is not the same for each weekday (indeed, to be the same, this number must be an integer), which is due to the effects of the "common" centennial years (1700, 1800, 1900, 2100, 2200, etc.).

The absence of an extra day in such years causes the following leap year (1704, 1804, 1904, 2104, etc.) to start on the same day of the week as the leap year twelve years before (1692, 1792, 1892, 2092 etc.). Similarly, the leap year eight years after a "common" centennial year (1708, 1808, 1908, 2108, etc.) starts on the same day of the week as the leap year immediately prior to the "common" centennial year (1696, 1796, 1896, 2096 etc.). Thus, those days of the week on which such leap years begin gain an extra year or two in each cycle.

The following table shows the distribution of extra days during each 400-year cycle:
Occurrences Leap year starts on Leap day occurs on
15 Sunday
Leap year starting on Sunday
This is the calendar for any leap year starting on Sunday, January 1 , such as 1956, 1984, 2012, 2040, or 2068.This is the only leap year with three occurrences of Friday the 13th, each three months apart in January, April, and July....

Wednesday
13 Monday
Leap year starting on Monday
This is the calendar for any leap year starting on Monday, January 1 , such as 1940, 1968, 1996, 2024 or 2052.MillenniumCenturyYear2nd Millennium:18th century:  1720  1748  1776...

Thursday
14 Tuesday
Leap year starting on Tuesday
This is the calendar for any leap year starting on Tuesday, January 1 , such as 1952, 1980, 2008, 2036 or 2064.Previous year | Next yearMillenniumCenturyYear2nd Millennium:18th century:  1760  1788...

Friday
14 Wednesday
Leap year starting on Wednesday
This is the calendar for any leap year starting on Wednesday, January 1 , such as 1936, 1964, 1992, 2020 or 2048.This kind of year has 53 weeks in the ISO 8601 week - day format.Previous year | Next yearMillenniumCenturyYear...

Saturday
13 Thursday
Leap year starting on Thursday
This is the calendar for any leap year starting on Thursday, January 1 , such as 1948, 1976, 2004, 2032 or 2060.Previous year | Next year...

Sunday
15 Friday
Leap year starting on Friday
This is the calendar for any leap year starting on Friday, January 1 , such as 1932, 1960, 1988, 2016 or 2044.Previous year | Next yearMillenniumCenturyYear2nd Millennium:18th century:17681796...

Monday
13 Saturday
Leap year starting on Saturday
This is the calendar for any leap year starting on Saturday, January 1 , such as 1972, 2000, 2028 or 2056.Previous year | Next year-Trivia:...

Tuesday


Note that as a cycle, this pattern is symmetric with respect to the low Saturday value.

A leap year starting on Sunday means the next year does not start on Monday, so more leap years starting on Sunday means fewer years starting on Monday, etc. Thus the pattern of number of years starting on each day is inverted and shifted by one weekday: 58, 56, 58, 57, 57, 58, 56 (symmetric with respect to the high Sunday value).

The number of common years starting on each day is found by subtraction: 43, 43, 44, 43, 44, 43, 43.

The frequency of a particular date being on a particular weekday can easily be derived from the above (for dates in March and later, relate them to the next New Year).

See also the cycle of Doomsdays.

Accuracy


The Gregorian calendar improves the approximation made by the Julian calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

 by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 mean solar day
Solar time
Solar time is a reckoning of the passage of time based on the Sun's position in the sky. The fundamental unit of solar time is the day. Two types of solar time are apparent solar time and mean solar time .-Introduction:...

s long. This approximation has an error of about one day per 3,300 year
Year
A year is the orbital period of the Earth moving around the Sun. For an observer on Earth, this corresponds to the period it takes the Sun to complete one course throughout the zodiac along the ecliptic....

s with respect to the mean tropical year. However, because of the precession of the equinoxes, the error with respect to the vernal equinox (which occurs, on average, 365.24237 days apart near 2000) is 1 day every 7,700 years. By any criterion, the Gregorian calendar is substantially more accurate than the 1 day in 128 years error of the Julian calendar (average year 365.25 days).

In the 19th century, Sir John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

 proposed a modification to the Gregorian calendar with 969 leap days every 4000 years, instead of 970 leap days that the Gregorian calendar would insert over the same period. This would reduce the average year to 365.24225 days. Herschel's proposal would make the year 4000, and multiples thereof, common instead of leap. While this modification has often been proposed since, it has never been officially adopted.

On time scales of thousands of years, the Gregorian calendar falls behind the astronomical seasons because the slowing down of the Earth's rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 makes each day slightly longer over time (see tidal acceleration
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...

 and leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

) while the year maintains a more uniform duration. Borkowski reviewed mathematical models in the literature, and found that the results generally fall between a model by McCarthy and Babcock, and another by Stephenson and Morrison. If so, in the year 4000, the calendar will fall behind by at least 0.8 but less than 1.1 days. In the year 12,000 the calendar would fall behind by at least 8 but less than 12 days.

Calendar seasonal error


This image shows the difference between the Gregorian calendar and the astronomical seasons.

The y-axis is the date in June and the x-axis is Gregorian calendar years.

Each point is the date and time of the June Solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

 (or Winter solstice
Winter solstice
Winter solstice may refer to:* Winter solstice, astronomical event* Winter Solstice , former band* Winter Solstice: North , seasonal songs* Winter Solstice , 2005 American film...

 in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

) on that particular year. The error shifts by about a quarter of a day per year. Centurial years are ordinary years, unless they are divisible by 400, in which case they are leap years. This causes a correction on years 1700, 1800, 1900, 2100, 2200, and 2300.

For instance, these corrections cause 23 December 1903 to be the latest December solstice, and 20 December 2096 to be the earliest solstice—2.25 days of variation compared with the seasonal event.

Leap seconds and other aspects


Since 1972, some years may also contain one or more leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

s, to account for cumulative irregularities in the Earth's rotation. So far, these have always been positive and have occurred on average once every 18 months.

The day of the year is somewhat inconvenient to compute, partly because the leap day does not fall at the end of the year. But the calendar exhibits a repeating pattern for the number of days in the months March through July and August through December: 31, 30, 31, 30, 31, for a total of 153 days each. In fact, any five consecutive months not containing February contain exactly 153 days.

See also common year starting on Sunday
Common year starting on Sunday
This is the calendar for any common year starting on Sunday, January 1 or for any year in which “Doomsday” is Tuesday. Examples: Gregorian years 1989, 1995, 2006, 2017 and 2023or Julian year 1917...

 and dominical letter
Dominical letter
Dominical letters are letters A, B, C, D, E, F and G assigned to days in a cycle of seven with the letter A always set against 1 January as an aid for finding the day of the week of a given calendar date and in calculating Easter....

.

The 400-year cycle of the Gregorian calendar has 146,097 days and hence exactly 20,871 weeks. So, for example, the days of the week in Gregorian 1603 were exactly the same as for 2003. The years that are divisible by 400 begin on a Saturday. In the 400-year cycle, more months begin on a Sunday (and hence have Friday the 13th
Friday the 13th
Friday the 13th occurs when the thirteenth day of a month falls on a Friday, which superstition holds to be a day of bad luck. In the Gregorian calendar, this day occurs at least once, but at most three times a year...

) than any other day of the week (see above under Week for a more detailed explanation of how this happens). 688 out of every 4800 months (or 172/1200) begin on a Sunday, while only 684 out of every 4800 months (171/1200) begin on each of Saturday and Monday, the least common cases.

A smaller cycle is 28 years (1,461 weeks), provided that there is no dropped leap year in between. Days of the week in years may also repeat after 6, 11, 12, 28 or 40 years. Intervals of 6 and 11 are only possible with common years, while intervals of 28 and 40 are only possible with leap years. An interval of 12 years only occurs with common years when there is a dropped leap year in between.

The Doomsday algorithm is a method to discern which of the 14 calendar variations should be used in any given year (after the Gregorian reformation). It is based on the last day in February, referred to as the Doomsday.

Day of the week for a date in different years


Common years always begin and end on the same day of the week, since 365 is one more than a multiple of 7 (52 [number of weeks in a year] × 7 [number of days in a week] = 364). For example, 2003 began on a Wednesday and ended on a Wednesday. Leap years end on the next day of the week from which they begin. For example, 2004 began on a Thursday and ended on a Friday.

Not counting leap years, any calendar date will move to the next day of the week the following year. For example, if a birthday fell on a Tuesday in 2002, it fell on a Wednesday in 2003. Leap years make things a little more complicated, and move any given date occurring after March two days in the week on the following year, "leaping over" an extra day, hence the term leap year. For example, 2004 was a leap year, so calendar days of 1 March or later in the year, moved two days of the week from 2003.

Calendar days occurring before 1 March do not make the extra day of the week jump until the year following a leap year. So, if a birthday is 15 June, then it must have fallen on a Sunday in 2003 and a Tuesday in 2004. If, however, a birthday is 15 February, then it must have fallen on a Saturday in 2003, a Sunday in 2004 and a Tuesday in 2005.

Calendar matches between months


In any year (even a leap year), July always begins on the same day of the week that April does; therefore, the only difference between a July calendar page and an April calendar page in the same year is the extra day July has. The same relationship exists between September and December, as well as between March and November: if an extra day is added to the September calendar, the calendar for December is obtained; remove a day from the March calendar, and that for November is obtained.

In common years (non-leap years), there are additional matches: October duplicates January, and March and November duplicate February in their first 28 days. In leap years only, there is a different set of additional matches: July is a duplicate of January, while February is duplicated in the first 29 days of August.

English names for year numbering system



The Anno Domini (Latin for "in the year of the/our Lord") system of numbering years, in which the leap year rules are written, and which is generally used together with the Gregorian calendar, is also known in English as the Common Era or Christian Era. Years before the beginning of the era are known in English as Before Christ, Before the Common Era, or Before the Christian Era. The corresponding abbreviations AD, CE, BC, and BCE are used. There is no year 0; AD 1 immediately follows 1 BC.

Naturally, since Inter gravissimas
Inter gravissimas
Inter gravissimas was a papal bull issued by Pope Gregory XIII on February 24, 1582. The document reformed the Julian calendar and created a new calendar which came to be called the Gregorian calendar, which is used in most countries today.-Description:...

was written in Latin, it does not mandate any English language nomenclature. Two era names occur within the bull, "anno Incarnationis dominicæ" ("in the year of the Incarnation of the Lord") for the year it was signed, and "anno à Nativitate Domini nostri Jesu Christi" ("in the year from the Nativity of our Lord Jesus Christ") for the year it was printed. Nevertheless, "anno Domini" and its inflections "anni Domini" and "annus Domini" are used many times in the canons attached to the bull.

Proposed reforms


The following are proposed reforms
Calendar reform
A calendar reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar.Most calendars have several rules which could be altered by reform:...

 of the Gregorian calendar:
  • Holocene calendar
    Holocene calendar
    The Human Era, also known as the Holocene calendar or Holocene era , is a year numbering system that adds exactly 10,000 years to the currently world-dominant Anno Domini and Common Era system, placing its first year near the beginning of the Holocene epoch and the Neolithic revolution...

  • International Fixed Calendar
    International Fixed Calendar
    The International Fixed calendar is a solar calendar proposal for calendar reform designed by Moses B...

     (also called the International Perpetual calendar)
  • World Calendar
    World calendar
    The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930.-Features:The World Calendar is a 12-month, perennial calendar with equal quarters. It is perennial, or perpetual, because it remains the same every year.Each quarter begins...

  • World Season Calendar
  • Leap week calendar
    Leap week calendar
    A leap week calendar is a calendar system with a whole number of weeks every year, and with every year starting on the same weekday. Most leap week calendars are proposed reforms to the civil calendar, but some - such as the ISO week number calendar - are simply conveniences for specific...

    s
    • Pax Calendar
      Pax Calendar
      The Pax calendar was invented by James A. Colligan in 1930 as a reform of the Gregorian calendar.Unlike other proposals such as the International Fixed Calendar and the World Calendar, it preserves the 7-day week by intercalating a week to a perpetual year of 52 weeks = 364 days.The year is divided...

    • Common-Civil-Calendar-and-Time
      Common-Civil-Calendar-and-Time
      The Common-Civil-Calendar-and-Time Calendar is a proposal for calendar reform. It is one of many examples of leap week calendars, calendars which maintain synchronization with the solar year by intercalating entire weeks rather than single days....

    • Symmetry454
      Symmetry454
      The Symmetry454 Calendar is a proposal for Gregorian calendar reform. It is a perpetual solar calendar that conserves the traditional 7-day week, has symmetrical equal quarters, and starts every month on Monday.-References:...


See also

  • Calendar reform
    Calendar reform
    A calendar reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar.Most calendars have several rules which could be altered by reform:...

  • Computus
    Computus
    Computus is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age....

     — Gregorian lunar calendar
  • Doomsday rule
  • Dual dating
    Dual dating
    In historical materials, dates will often be indicated with what appears to be duplicate, or excessive digits, sometimes separated by a hyphen or a slash. This is often referred to as double dating. The need for double dating arose from the transition from an older calendar to a newer one...

  • Inter gravissimas in English — Wikisource
  • Julian day calculation
  • List of calendars
  • Old Style and New Style dates
    Old Style and New Style dates
    Old Style and New Style are used in English language historical studies either to indicate that the start of the Julian year has been adjusted to start on 1 January even though documents written at the time use a different start of year ; or to indicate that a date conforms to the Julian...

  • Old Calendarists
    Old calendarists
    The term Old Calendarist refers to any Orthodox Christian or any Orthodox Church body which uses the historic Julian calendar , and whose Church body is not in communion with the Orthodox Churches that use the New Calendar...

    • Greek Old Calendarists
      Greek Old Calendarists
      Greek Old Calendarists are groups that separated from the Orthodox Church of Greece or from the Patriarchate of Constantinople, precipitated by disagreement over the abandonment of the traditional Julian Calendar.- History :Up until the early 20th century, the Eastern Orthodox Church used the...


External links