Epoch (reference date)

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Epoch (reference date)

In the fields of chronology

Chronology

Chronology is a chronicle or arrangement of events in their occurrence order. General chronology is the science of locating and resolution of temporal sequence of past events in time...
and periodization

Periodization

Era

An era is a commonly used word for long period of time. When used in science, for example geology, eras denote clearly defined periods of time of arbitrary but well defined length, such as for example the Mesozoic era from 252 Ma?66 Ma, delimited by a start event and an end event....
. The "epoch" then serves as a reference point from which time is measured. Time measurement units

Time

Time is a component of the measurement used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects....
are counted from the epoch so that the date and time of events can be specified unambiguously.

Events taking place before the epoch can be dated by counting negatively from the epoch, though in pragmatic periodization

Periodization

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Encyclopedia

In the fields of chronology

Chronology

Periodization

Era

Time

Periodization

Periodization is the attempt to categorize or divide time into named blocks. The result is a descriptive abstraction that provides a useful handle on periods of time with relatively stable characteristics....
practice, epochs are defined for the past, and another epoch is used to start the next era, therefore serving as the ending of the older preceding era. The whole purpose and criteria of such definitions is to clarify and co-ordinate scholarship about a period, at times, across disciplines.

Epochs are generally chosen to be convenient or significant by a consensus of the time scale's initial users, or by authoritarian fiat. The epoch moment or date is usually defined by a specific clear event, condition, or criteria— the epoch event or epoch criteria —from which the period or era or age is usually characterized or described.

Examples:

by events: The assassination of the Roman Emperor

Roman Emperor

The Roman Emperor was the ruler of the Roman Empire during the imperial period . The Romans had no single term for the office: Latin language titles such as imperator , Augustus , Caesar and princeps were all associated with it....
Alexander Severus

Alexander Severus

Marcus Aurelius Severus Alexander, commonly called Alexander Severus, was the last Roman Emperors of the Severan dynasty, having succeeded, as heir apparent, his despised cousin, the eighteen year old Elagabalus who had been murdered along with his mother by his own guards—and as a mark of contempt, had their remains cast into...
triggering the Crisis of the Third Century

Crisis of the Third Century

Crisis of the Third Century was the crumbling and near collapse of the Roman Empire between 235 and 284 caused by invasion, civil war, Plague of Cyprian, and economic collapse....
The defenestration of Prague

Defenestrations of Prague

The Defenestrations of Prague were two incidents in the history of Bohemia. The first occurred in 1419 and the second in 1618, although the term "Defenestration of Prague" is more commonly used to refer to the second incident....
triggering the horrible depopulation of Europe and the Thirty Years' War

Thirty Years' War

The Thirty Years' War was one of the most destructive conflicts in European history. The war was fought primarily in Germany and at various points involved most of the countries of Europe....
Queen Victoria ascending to the throne giving the start of the Victorian era

Victorian era

The Victorian Era of the United Kingdom of Great Britain and Ireland was the period of Victoria of the United Kingdom reign from June 1837 to January 1901....

by criteria: The spurt in exploration, mercantilism, and colonization in the Age of Discovery

Age of Discovery

The Age of Discovery, also known as the Age of Exploration, was a period in human history starting in the 15th Century and continuing into the 17th Century, during which Europeans explored the world by ocean searching for trading partners and particular trade goods....
Particular ratios of animal fossils in a rock strata —various Geology epochs

Calendars

Each calendar era

Calendar era

A calendar era is the year numbering system used by a calendar. For example, the Gregorian calendar numbers its years in the Western Christian era ....
starts from an arbitrary epoch, which is often chosen to commemorate an important historical or mythological event. For example, the epoch of the anno Domini

Anno Domini

, abbreviated as 'AD' or 'A.D.', and 'Before Christ', abbreviated as 'BC' or 'B.C.', are designations used to number years in the Julian calendar and Gregorian calendars....
calendar era (the civil calendar era used internationally and in many countries) is the traditionally-reckoned Incarnation

Incarnation (Christianity)

The Incarnation is the belief in Christianity that Jesus Christ is God in human body. The word Incarnate derives from Latin meaning "in the flesh." The incarnation is a fundamental theological teaching of Nicene Creed, based on its understanding of the New Testament....
of Jesus

Jesus

Jesus of Nazareth , also known as Jesus Christ, is the central figure of Christianity and is revered by most Christian churches as the Son of God and the Incarnation ....
. Many other current and historical calendar eras

Common Era

Common Era, abbreviated as CE, is a designation for the calendar system most commonly used in the Western world, and also internationally, for numbering the year part of the calendar date....
exist, each with its own epoch.

Asian national eras

The official Japanese system
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....
numbers years from the accession of the current emperor
Emperor of Japan

The of Japan is the symbol of the state and of the unity of the Japanese people. He is the head of the Imperial House of Japan. Under Japan's present constitution, the Emperor is the "symbol of the state and the unity of the people," and is a ceremonial figurehead in a constitutional monarchy ....
, regarding the calendar year during which the accession occurred as the first year.
A similar system
Chinese era name

A Chinese era name is the era name, reign period, or regnal title used when traditionally numbering years in an emperor's reign and naming certain Chinese rulers ....
existed in China
China

China is a Culture of China, an ancient civilization, and, depending on perspective, a national or multinational entity extending over a large area in East Asia....
before 1912, being based on the accession year of the emperor (1911 was thus the fourth year of the Xuantong period). With the establishment of the Republic of China
Republic of China

The Republic of China , also known as Nationalist China is a country in East Asia that has evolved from a single-party state with full global recognition into a multi-party democratic state with Political status of Taiwan....
in 1912, the republican era was introduced. It is still very common in Taiwan
Taiwan

Taiwan is an island in East Asia. "Taiwan" is also commonly used to refer to the country governed by the Republic of China and to the ROC itself, which governs the island of Taiwan, Orchid Island and Green Island, Taiwan in the Pacific Ocean off the Taiwan coast, the Penghu islands in the Taiwan Strait, and Kinmen and the Matsu Islands...
to date events via the republican era. The People's Republic of China adopted the common era calendar in 1949 (the 38th year of the Chinese Republic).
North Korea
North Korea

North Korea, officially the Democratic People's Republic of Korea , is a state in East Asia, occupying the northern half of the Korean Peninsula....
uses a system that starts in 1912 (= Juche
Juche

The Juche Idea is the official state ideology of North Korea and the political system based on it. The doctrine is a component part of Kimilsungism, the North Korean term for Kim Il-sung's family regime....
1), the year of the birth of their founder Kim Il-Sung
Kim Il-sung

Kim Il-sung was the president and absolute ruler of North Korea from its founding in early 1948 until his death, when he was succeeded by his son Kim Jong-il....
. The year 2009 is "Juche 98". Juche means "autarky
Autarky

An autarky is an Economics that is Self-sufficiency and does not take part in international trade, or severely limits trade with the outside world....
, self-reliance".
In Thailand
Thailand

The Kingdom of Thailand is an independent country that lies in the heart of Southeast Asia. It is bordered to the north by Laos and Myanmar, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the west by the Andaman Sea and Myanmar....
in 1888 King Chulalongkorn
Chulalongkorn

Phrabat Somdet Phra Poramintramaha Chulalongkorn, Phra Chulachomklao Chaoyuhua was the fifth monarch of the Chakri dynasty. He was known to the Siamese of his time as Phra Buddhachao Luang ....
decreed a National Thai Era dating from the founding of Bangkok
Bangkok

The city of Bangkok is the Capital , largest urban area and primary city of Thailand. Known in Thai language as Krung Thep Maha Nakhon or Krung Thep for short, it was a small trading post at the mouth of the Chao Phraya River during the Ayutthaya Kingdom and came to the forefront of Thailand when it was given the status as the...
on April 6, 1782. In 1912, New Year's Day was shifted to April 1. In 1941, Prime Minister Phibunsongkhram
Plaek Pibulsonggram

Field Marshal Thai royal and noble titles#Luang Plaek Pibulsonggram was List of Prime Ministers of Thailand and military dictator of Thailand from 1938 to 1944 and 1948 to 1957....
decided to count the years since 543 BC. This is the Thai solar calendar
Thai solar calendar

The Thai solar calendar, Suriyakati , has been the official and prevalent calendar in Thailand since it was adopted by King Chulalongkorn in 1888, although the Western calendar year is sometimes used in business, and quite often in banking....
using the Thai Buddhist Era. Except for this era, it is the Gregorian calendar.

Religious eras

In Israel
Israel

Israel officially the State of Israel , is a country in the Middle East located on the eastern shore of the Mediterranean Sea. It borders Lebanon in the north, Syria in the northeast, Jordan in the east, and Egypt on the southwest, and contains geographically diverse features within its relatively small area....
, the traditional Hebrew calendar
Hebrew calendar

The Hebrew calendar or Jewish calendar is a lunisolar calendar used by Jews, now predominantly for religious purposes. It is used to reckon the Jewish New Year and dates for Jewish holidays, and also to determine appropriate Torah reading of Torah portions, Yahrzeits , and daily Psalm reading, among many ceremonial uses....
, using an era dating from Creation
Anno Mundi

File:Rotunda Yard Thessaloniki 05 Jew Tomb remains.JPG abbreviated as 'AM' or 'A.M.', refers to a Calendar era counting from the Bible Creation according to Genesis of the world....
, is the official calendar. However, the Gregorian calendar is the de facto calendar and is commonly used. Government documents usually display a dual date. The beginning of year 1 of the Hebrew calendar occurred in the autumn of 3761 BC. Therefore, "Rosh Hashanah, the Jewish New Year, in September 2003 marked the transition from 5763 to 5764".
In the Islam
Islam

Islam is a Monotheism, Abrahamic religion originating with the teachings of the Prophets of Islam Muhammad, a 7th century Arab religious and political figure....
ic world, traditional Islamic dating
Islamic calendar

The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar used to date events in many predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals....
according to the Anno Hegiræ (in the year of the hijra
Hijra (Islam)

The Hijra is the migration of the Islamic prophet Muhammad and his followers to the city of Medina in 622 . Alternate spellings of this Arabic language word in the Latin alphabet are Hijrah, or Hegira in Latin....
) or AH era remains in use to a varying extent, especially for religious purposes. The official Iranian calendar
Iranian calendar

The Iranian calendar or Solar Hejri is an astronomical solar calendar and one of the longest chronological records in history and is currently used in Iran and Afghanistan as the main official calendar....
(used in Afghanistan
Afghanistan

Afghanistan , officially the Islamic republic of Afghanistan, is a landlocked country that is located approximately in the center of Asia....
as well as Iran
Iran

Iran , officially the Islamic Republic of Iran and formerly known internationally as Persian Empire until 1935, is a country in Central Eurasia, located on the northeastern shore of the Persian Gulf and the southern shore of the Caspian Sea....
) also dates from the hijra, but as it is a solar calendar
Solar calendar

A solar calendar is a calendar whose dates indicate the position of the earth on its revolution around the sun ....
its year numbering does not coincide with the religious calendar.

Other

In the French Republican Calendar
French Republican Calendar

The French Republican Calendar or French Revolutionary Calendar was a calendar proposed during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days in 1871 in Paris....
, a calendar used by the French government for about twelve years from late 1793, the epoch was the beginning of the "Republican Era", September 22, 1792 (the day the French First Republic
French First Republic

The French First Republic was founded on 22 September, 1792, by the newly established National Convention. The First Republic lasted until the declaration of the First French Empire in 1804 under Napoleon....
was proclaimed, one day after the Convention abolished the monarchy).
In the scientific Before Present
Before Present

Before Present years are a time scale used in archaeology, geology, and other science disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use 1950 Common_Era as the arbitrary origin of the age scale....
system of numbering years for purposes of radiocarbon dating
Radiocarbon dating

Radiocarbon dating, or carbon dating, is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to determine the age of carbonaceous materials up to about 60,000 years....
, the reference date is January 1, 1950 (though the use of January 1 is mostly irrelevant, as radiocarbon dating is approximate to years and days can rarely be calculated accurately).
Different branches of Freemasonry
Freemasonry

Freemasonry is a fraternal and service organizations that arose from obscure origins in the late 16th to early 17th century. Freemasonry now exists in various forms all over the world, with a membership estimated at around 5 million ....
have selected different years to date their documents according to a Masonic era, such has the Anno Lucis (A.L.).

Astronomy

In astronomy, an epoch

Epoch (reference date)

In the fields of chronology and periodization, an epoch means an instant in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured....
is a specific moment in time for which celestial coordinates

Celestial coordinate system

In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky.There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth....
or orbital elements

Orbital elements

In celestial mechanics, the elements of an orbit are the parameters needed to specify that orbit uniquely. Orbital elements are generally considered in classical mechanics two-body systems, where a Kepler orbit is used ....
are specified, and from which other orbital parametrics are thereafter calculated in order to predict future position. The applied tools of the mathematics disciplines of Celestial mechanics

Celestial mechanics

Celestial mechanics is the branch of astronomy that deals with the motion s of celestial objects. The field applies principles of physics, historically classical mechanics, to astronomical objects such as stars and planets to produce ephemeris data....
or its subfield Orbital mechanics (both predict orbital paths and positions) about a center of gravity are used to generate an ephemeris

Ephemeris

An ephemeris is a table of values that gives the positions of astronomical objects in the sky at a given time or times. Different kinds are used for astronomy and astrology....
(plural: ephemerides; from the Greek word ephemeros = daily) which is a table of values that gives the positions of astronomical objects in the sky at a given time or times, or a formula to calculate such given the proper time offset from the epoch. Such calculations generally result in an elliptical path on a plane defined by some point on the orbit, and the two focii of the ellipse

Ellipse

In mathematics, an ellipse is the apparent shape of a circle viewed obliquely from outside it, as distinct from a hyperbola which is the shape seen from inside....
. Viewing from another orbiting body, following its own trace and orbit, creates shifts in three dimensions in the spherical trigonometry used to calculate relative positions. Interestingly, these dynamics in three dimensions are also elliptical, which means the ephemeris need only specify one set of equations to be a useful predictive tool to predict future location of the object of interest.

Over time, inexactitudes and other errors accumulate, creating more and greater errors of prediction, so ephemeris factors need recalculated from time to time, and that requires a new epoch to be defined. Different astronomers or groups of astronomers used to define epochs to suit themselves, but these days of speedy communications, the epochs are generally defined in an international agreement, so astronomers world wide can collaborate more effectively. It was inefficient and error prone for data observed by one group to need translation (mathematic transformation) so other groups could compare information.

J2000.0

The current standard epoch is called "J2000.0" (Julian 2000, based on the Julian Calendar

Julian calendar

The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC, and came into force in 45 BC . It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus....
), is defined by international agreement, and is precisely defined to be approximately noon at the Royal Observatory

Royal Observatory, Greenwich

The Royal Observatory, Greenwich was commissioned in 1675 by Charles II of England, with the foundation stone being laid on 10 August. At this time the king also created the position of Astronomer Royal , to serve as the director of the observatory and to "apply himself with the most exact care and diligence to the rectifying of the tab...
, Greenwich

Greenwich

'Greenwich' is a district in south-east London, England, on the south bank of the River Thames in the London Borough of Greenwich. It is best known for its maritime history and as giving its name to the Greenwich Meridian and Greenwich Mean Time....
, in London

London

London is the capital of both England and the United Kingdom, and the most populous municipality in the European Union. An important settlement for two millennia, History of London goes back to its founding by the Roman Empire....
England

England

The Julian date 2451545.0 TT (Terrestrial Time
Terrestrial Time

Terrestrial Time is the modern astronomical time standard for the passage of time on the surface of the Earth . Since time moves at different rates for observers in different locations , and "the surface of the Earth" is not a single point in space, TT is a theoretical ideal; its measurement is approximated by the International Atomic Time...
), or January 1, 2000, noon TT.
This is equivalent to January 1, 2000, 11:59:27.816 TAI (International Atomic Time
International Atomic Time

International Atomic Time is a high-precision atomic coordinate time time standard based on the notional passage of proper time on Earth's geoid....
) or
January 1, 2000, 11:58:55.816 UTC (Coordinated Universal Time
Coordinated Universal Time

Coordinated Universal Time is a time standard based on International Atomic Time with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation....
).

Computing

The time kept internally by a computer

Computer

A computer is a machine that manipulates Data according to a list of Code .The first devices that resemble modern computers date to the mid-20th century , although the computer concept and various machines similar to computers existed earlier....
system is usually expressed as the number of time units that have elapsed since a specified epoch, which is nearly always specified as midnight Universal Time

Universal Time

Universal Time is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the meridian of Royal Observatory, Greenwich, and GMT is sometimes used loosely as a synonym for UTC....
on some particular date.

Software timekeeping systems vary widely in the granularity of their time units; some systems may use time units as large as a day, while others may use nanoseconds. For example, for an epoch date of midnight UTC on January 1, 1900, and a time unit of a second, the time of midnight UTC on January 2, 1900 is represented by the number 86400, the number of seconds in one day. When times prior to the epoch need to represented, it is common to use the same system, but with negative numbers.

These representations of time are mainly for internal use. If an end user interaction with dates and times is required, the software will nearly always convert this internal number into a date and time representation that is comprehensible to humans.

Notable epoch dates in computing

The following table lists epoch dates used by popular software and other computer-related systems. The time in these systems is stored as the number some time unit (days, seconds, nanoseconds, etc) that have elapsed since midnight UTC on the given date.

epoch date	notable uses	rationale for selection
January 1, 0 Year zero Year zero is not used in the widely used Gregorian calendar, nor in its predecessor, the Julian calendar. Under those systems, the year 1 BC is followed by AD 1....	matlab MATLAB MATLAB is a Numerical analysis environment and programming language. Maintained by The MathWorks, MATLAB allows easy matrix manipulation, plotting of function and data, implementation of algorithms, creation of user interfaces, and interfacing with programs in other languages.... , Turbo DB and tdbengine Tdbengine Tdbengine is a relational database management system with an integrated programming language. It is designed to handle databases on the web. It connects to the webserver using the standard Common Gateway Interface interface....
January 1, 1	Symbian, Microsoft .NET, REXX REXX REXX is an Interpreted language programming language which was developed at IBM. It is a structured high-level programming language which was designed to be both easy to learn and easy to read....	This epoch date is known as Rata Die Rata Die Rata Die is a system for assigning numbers to calendar days , independent of any calendar, for the purposes of calendrical calculations.It was named by Edward Reingold and Nachum Dershowitz for their book Calendrical Calculations....
January 1, 1601	NTFS NTFS NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7.... , COBOL COBOL COBOL is one of the oldest programming languages still in active use. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments.... , Win32/Win64	1601 was the first year of the 400-year Gregorian calendar cycle at the time Windows NT was made.
December 31, 1840	MUMPS MUMPS MUMPS , or alternatively M, is a programming language created in the late 1960s, originally for use in the Health care. It was designed for the production of multi-user database-driven applications.... programming language	1841 was roughly the birthyear of the world's oldest living person when the language was designed.
November 17, 1858	VMS OpenVMS OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is the name of a high-end computer server operating system that runs on the VAX and DEC Alpha families of computers, developed by Digital Equipment Corporation of Maynard, Massachusetts, Massachusetts , and most recently on Hewlett-Packard systems built around the In... , United States Naval Observatory United States Naval Observatory The United States Naval Observatory is one of the oldest scientific agencies in the United States. Located in Northwest, Washington, D.C. Washington, D.C., it is one of the few observatory located in an urban area; at the time of its construction, it was far from the light pollution generated by the city center.... , other astronomy Astronomy Astronomy is the science of Astronomical object and Phenomenon that originate outside the Earth's atmosphere . It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the physical cosmology.... -related computations	November 17, 1858 equals the Julian Day 2,400,000.
December 30, 1899	Microsoft COM DATE Microsoft Excel Microsoft Excel is a spreadsheet-application written and distributed by Microsoft for Microsoft Windows and Mac OS X. It features calculation, graphing tools, pivot tables and a macro programming language called VBA ....	Technical internal value used by Microsoft Excel; to simplify calculations by falsely assuming 1900 to be a leap year.
January 0, 1900	Microsoft Excel Microsoft Excel Microsoft Excel is a spreadsheet-application written and distributed by Microsoft for Microsoft Windows and Mac OS X. It features calculation, graphing tools, pivot tables and a macro programming language called VBA .... , Lotus 1-2-3 Lotus 1-2-3 Lotus 1-2-3 is a spreadsheet program from Lotus Software . It was the IBM PC's first "killer application"; its huge popularity in the mid-1980s contributed significantly to the success of the IBM PC in the corporate environment....	While logically January 0, 1900 is equivalent to December 31, 1899, these systems do not allow users to specify the latter date.
January 1, 1900	Network Time Protocol Network Time Protocol The Network Time Protocol is a protocol for clock synchronization of computer systems over packet-switched, variable-Latency data networks. NTP uses User Datagram Protocol on TCP and UDP port 123 as its transport layer.... , IBM IBM International Business Machines Corporation, abbreviated IBM and nicknamed "Big Blue" , is a multinational corporation computer technology and consulting corporation headquartered in Armonk, New York, New York, United States.... CICS CICS CICS is a transaction server that runs primarily on IBM mainframe systems under z/OS and z/VSE.CICS is a transaction manager designed for rapid, high-volume online processing.... , Mathematica Mathematica Mathematica is a computational software program used widely in scientific, engineering, and mathematical fields and other areas of technical computing.... , RISC OS RISC OS RISC OS is a computer operating system which was originally developed by Acorn Computers Ltd in Cambridge, England for their ARM architecture based computers....
January 1, 1904	Apple Inc.'s Mac OS Mac OS Mac OS is the trademarked name for a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems.... through version 9, Palm OS Palm OS Palm OS is an embedded operating system operating system initially developed by U.S. Robotics Corp.-owned Palm, Inc. for personal digital assistants in 1996.... , MP4, Microsoft Excel (optionally)	1904 is the first leap year of the twentieth century.
January 1, 1960	S-Plus S-PLUS S-PLUS is a commercial advanced List of statistical packages sold by TIBCO Software Inc.. It is a commercial implementation of the S programming language.... , SAS SAS System SAS is an integrated system of software products provided by SAS Institute that enables the programmer to perform:*data entry, Information retrieval, Data management, and Data mining...
January 1, 1968	Pick OS Pick operating system The Pick operating system is a Demand paging, multiuser, virtual memory, time-sharing operating system based around a unique MultiValue. It is used primarily for business data processing....
January 1, 1970	Unix time Unix time Unix time, or POSIX time, is a system for describing points in time, defined as the number of seconds elapsed since midnight Coordinated Universal Time of January 1 1970, not counting leap seconds.... , used by UNIX Unix Unix is a computer operating system originally developed in 1969 by a group of American Telephone & Telegraph employees at Bell Labs, including Ken Thompson , Dennis Ritchie, Douglas McIlroy, and Joe Ossanna.... , Linux Linux Linux is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed by anyone under the terms of the GNU GPL license... , other UNIX-like systems, Mac OS X Mac OS X Mac OS X is a line of computer operating systems developed, marketed, and sold by Apple Inc., and since 2002 has been included with all new Macintosh computer systems.... , Java Java (programming language) Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java .... , JavaScript JavaScript JavaScript is a scripting language widely used for client-side web development. It was the originating Programming language dialect of the ECMAScript standard.... , PHP Programming Language PHP PHP is a scripting language originally designed for producing dynamic web pages. It has evolved to include a command line interface capability and can be used in Standalone software Graphical user interface....
January 1, 1978	AmigaOS AmigaOS AmigaOS is the default native operating system of the Amiga personal computer. It was developed first by Commodore International, and initially introduced in 1985 with the Amiga 1000....
January 1, 1980	MS DOS, OS/2 OS/2 OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "IBM Personal System/2 " line of second-generation personal computers.... , FAT16 File Allocation Table File Allocation Table or FAT is a computer file system architecture now widely used on most computer systems and most memory cards, such as those used with digital cameras.... and FAT32 File Allocation Table File Allocation Table or FAT is a computer file system architecture now widely used on most computer systems and most memory cards, such as those used with digital cameras.... filesystem
January 6, 1980	Qualcomm Qualcomm Qualcomm is a wireless telecommunications research and development company based in San Diego, California, California.Corporate history... BREW Binary Runtime Environment for Wireless BREW is an application development platform created by QUALCOMM for telephone. It was originally developed for CDMA handsets, but has since been ported to other air interfaces including GSM/GPRS.... , GPS Global Positioning System The Global Positioning System is a global navigation satellite system developed by the United States Department of Defense and managed by the United States Air Force 50th Space Wing....
January 1, 1981	Acorn NetFS
January 1, 2000	Various updated MS DOS legacy software	Retrofitting to address the Year 2000 problem Year 2000 problem The Year 2000 problem was a notable computer bug resulting from the practice in early computer program design of representing the year with two digits....
January 1, 2001	Apple's Cocoa (API) Cocoa (API) Cocoa is one of Apple Inc.'s native object-oriented application program environment for the Mac OS X operating system. It is one of four major Application programming interfaces available for Mac OS X; the others are Carbon , POSIX , and Java platform.... framework	2001 is the year of the release of Mac OS X 10.0.

Problems with epoch-based computer time representation

Computers don't generally store arbitrarily large numbers. Instead, each number stored by a computer is allotted a fixed amount of space. Therefore, when the number of time units that have elapsed since a system's epoch exceeds the largest number that can fit in the space allotted to the time representation, the time representation overflow

Arithmetic overflow

The term arithmetic overflow or simply overflow has the following meanings.# In a digital computer, the condition that occurs when a calculation produces a result that is greater in magnitude than what a given processor register or Computer storage location can store or represent....
s, and problems can occur. While a system's behavior after overflow occurs is not necessarily predictable, one likely result is that the number representing the time will reset to zero, and the computer system will think that the current time is the epoch time again.

Most famously, older systems which counted time as the number of years elapsed since the epoch of January 1, 1900 and which only allotted enough space to store the numbers 0 through 99, experienced the Year 2000 problem

Year 2000 problem

The Year 2000 problem was a notable computer bug resulting from the practice in early computer program design of representing the year with two digits....
. These systems (if not corrected beforehand) would interpret the date January 1, 2000 as January 1, 1900, leading to unpredictable errors at the beginning of the year 2000.

Even systems which allocate more storage to the time representation are not immune from this kind of error. Many UNIX

Unix

Unix is a computer operating system originally developed in 1969 by a group of American Telephone & Telegraph employees at Bell Labs, including Ken Thompson , Dennis Ritchie, Douglas McIlroy, and Joe Ossanna....
-like operating system

Operating system

An operating system is an interface between hardware and applications; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer....
s which keep time as seconds elapsed from the epoch date of January 1, 1970, and allot timekeeping enough storage to store numbers as large as will experience an overflow problem on January 19, 2038 if not fixed beforehand. This is known as the Year 2038 problem

Year 2038 problem

The year 2038 problem may cause some computer software to fail before or in the year 2038. The problem affects all software and systems that store system time as a integer , and interpret this number as the number of seconds since 00:00:00 January 1, 1970....
. A correction involving doubling the storage allocated to timekeeping on these systems will allow them to represent dates more than 290 billion years into the future.

Other more subtle timekeeping problems exist in computing, such as accounting for leap second

Leap second

A leap second is a plus or minus one-second adjustment to the Coordinated Universal Time time scale that keeps it close to 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, which are not observed with any predictability or regularity. Additionally, applications which need to represent historical dates and times (for example, representing a date prior to the switch from the Julian calendar

Julian calendar

Gregorian calendar

The Gregorian calendar is the internationally accepted civil calendar. It was first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, after whom it was named, on 24 February 1582 by the papal bull Inter gravissimas....
) must use specialized timekeeping libraries.

Finally, some software must maintain compatibility with older software that does not keep time in strictly accordance with traditional timekeeping systems. For example, Microsoft Excel

Microsoft Excel

Microsoft Excel is a spreadsheet-application written and distributed by Microsoft for Microsoft Windows and Mac OS X. It features calculation, graphing tools, pivot tables and a macro programming language called VBA ....
observes the fictional date of February 29, 1900 in order to maintain compatibility with older versions of Lotus 1-2-3

Lotus 1-2-3

Lotus 1-2-3 is a spreadsheet program from Lotus Software . It was the IBM PC's first "killer application"; its huge popularity in the mid-1980s contributed significantly to the success of the IBM PC in the corporate environment....
, which observed that date to simplify time calculations at a time when computers were much slower.

Geology

In geology

Geology

Geology is the science and study of the solid and liquid matter that constitute the Earth. The field of geology encompasses the study of the composition, structural geology, physical properties, dynamics, and History of the Earth of Earth materials, and the processes by which they are formed, moved, and changed....
, an Epoch is a time division criteria based on specific physical and chemical characteristics in rock layers (Lithography

Lithography

Lithography is a method for printing using a stone or a metal plate with a completely smooth surface. By contrast, in intaglio a plate is engraving, etching or mezzotint to make cavities to contain the printing ink, and in woodblock printing and letterpress ink is applied to the raised surfaces of letters or images....
) indicative of the global environment, including tectonic activity (mountain building, continental drift, etc.) and include macroscopic (visible by inspection) and microscopic Fauna

Fauna

File:Fauna.pngFauna is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoology and paleontology use fauna to refer to a typical collection of animals found in a specific time or place, e.g....
features or characteristics that are clear and distinct from other series defining criteria. For example, the formations, rock beds

Bed (geology)

In geology a bed is the smallest division of a geologic formation or stratigraphy rock series marked by well-defined divisional planes separating it from layers above and below....
, and members of differing rock types that were being laid down in different environments at the same time.

Each defined set of characteristics—in a word, Epochs —directly correspond to specific rock series, the equivalent of an "era" (both are measured usually in time units) in rock layers. Since such layers correspond to time (or era) of formation, they are used to date pre-historic events in other sciences.

External links

(J. R. Stockton), an extensive list of dates that are problematic for various operating systems and computing devices
A list of potential problem dates for computers and software from 2001 to 2100.