| 24-hour clock |
12-hour clockThe 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...
|
| 00:00 |
12:00 (start
of day)
"Midnight" |
| 01:00 |
1:00 a.m. |
| 02:00 |
2:00 a.m. |
| 03:00 |
3:00 a.m. |
| 04:00 |
4:00 a.m. |
| 05:00 |
5:00 a.m. |
| 06:00 |
6:00 a.m. |
| 07:00 |
7:00 a.m. |
| 08:00 |
8:00 a.m. |
| 09:00 |
9:00 a.m. |
| 10:00 |
10:00 a.m. |
| 11:00 |
11:00 a.m. |
| 12:00 |
12:00
Noon |
| 13:00 |
1:00 p.m. |
| 14:00 |
2:00 p.m. |
| 15:00 |
3:00 p.m. |
| 16:00 |
4:00 p.m. |
| 17:00 |
5:00 p.m. |
| 18:00 |
6:00 p.m. |
| 19:00 |
7:00 p.m. |
| 20:00 |
8:00 p.m. |
| 21:00 |
9:00 p.m. |
| 22:00 |
10:00 p.m. |
| 23:00 |
11:00 p.m. |
| 24:00 |
("Midnight")*
(end of day) |
* See "Confusion
at noon and midnight" |
The
24-hour clock is a convention of time keeping in which the
dayA day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
runs from midnight to midnight and is divided into 24
hourThe hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
s, indicated by the hours passed since midnight, from 0 to 23. This system is the most commonly used time notation in the world today. The
12-hour clockThe 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...
is, however, still dominant in a handful of countries, particularly in
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
,
CanadaCanada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
(except
QuebecQuebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....
),
IndiaIndia , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
,
IranIran , officially the Islamic Republic of Iran , is a country in Southern and Western Asia. The name "Iran" has been in use natively since the Sassanian era and came into use internationally in 1935, before which the country was known to the Western world as Persia...
, the
PhilippinesThe Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
,
PakistanPakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...
, the
United KingdomThe United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, and some Latin American nations. The 24-hour notation is also popularly referred to as
military time or
astronomical time in the United States and Canada. It is also the
international standardThe International Organization for Standardization , widely known as ISO, is an international standard-setting body composed of representatives from various national standards organizations. Founded on February 23, 1947, the organization promulgates worldwide proprietary, industrial and commercial...
notation of time (
ISO 8601ISO 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...
). In the practice of
medicineMedicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, the 24-hour clock is generally used in documentation of care as it prevents any ambiguity as to when events occurred in a patient's
medical historyThe medical history or anamnesis of a patient is information gained by a physician by asking specific questions, either of the patient or of other people who know the person and can give suitable information , with the aim of obtaining information useful in formulating a diagnosis and providing...
.
Description
A time of day is written in the 24-hour notation in the form hh:mm (for example 01:23) or hh:mm:ss (for example, 01:23:45), where hh (00 to 23) is the number of full hours that have passed since
midnightMidnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....
, mm (00 to 59) is the number of full minutes that have passed since the last full hour, and ss (00 to 59) is the number of seconds since the last full minute. In the case of a
leap secondA 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...
, the value of ss may extend to 60. A leading zero is added for numbers under 10. This zero is optional for the hours, but very commonly used in computer applications, where many specifications require it (for example,
ISO 8601ISO 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...
). Where subsecond resolution is required, the seconds can be a decimal fraction, that is, the fractional part follows a decimal dot or comma, as in 01:23:45.678. The most commonly used separator symbol between hours, minutes and seconds is the
colonThe colon is a punctuation mark consisting of two equally sized dots centered on the same vertical line.-Usage:A colon informs the reader that what follows the mark proves, explains, or lists elements of what preceded the mark....
, which is also the symbol used in ISO 8601. In the past, some European countries used the dot on the line as a separator, but most national standards on time notation have since then been changed to the international standard colon. In some contexts (e.g., U.S. military, some computer protocols), no separator is used (e.g., 2359) and in some jurisdictions (France and Quebec) the letter
h (for "heure") is used when indicating hours and minutes only (18h45).
Midnight 00:00 and 24:00
In the 24-hour time notation, the day begins at
midnightMidnight is the transition time period from one day to the next: the moment when the date changes. In the Roman time system, midnight was halfway between sunset and sunrise, varying according to the seasons....
, 00:00, and the last minute of the day begins at 23:59. Where convenient, the notation 24:00 may also be used to refer to midnight at the end of a given date—that is, 24:00 of one day is the same time as 00:00 of the following day.
The notation 24:00 mainly serves to refer to the exact end of a day in a time interval. A typical usage is giving opening hours ending at midnight (e.g. "00:00–24:00", "07:00–24:00"). Similarly, some railway timetables show 00:00 as departure time and 24:00 as arrival time. Legal contracts often run from the start date at 00:00 till the end date at 24:00. It should be stressed, however, that "24:00" is a notation for the purposes of clarity and does not represent the display seen on the face of a clock.
While the 24-hour notation does unambiguously distinguish between midnight at the start (00:00) and end (24:00) of any given date, there is no such commonly accepted distinction among users of the 12-hour notation. Therefore,
style guideA style guide or style manual is a set of standards for the writing and design of documents, either for general use or for a specific publication, organization or field...
s and military communication regulations in some English-speaking countries discourage the use of 00:00 and 24:00 even in the 24-hour notation, and recommend reporting times near midnight as 23:59 or 00:01 instead, to avoid misunderstandings when such times are converted into the 12-hour notation later.
Time-of-day notations beyond 24:00 (such as 24:01 or 25:59 instead of 00:01 or 01:59) are not commonly used and not covered by the relevant standards. However, they have been observed occasionally in some special contexts in Japan and Hong Kong where business hours extend beyond midnight, such as broadcast-television production. They also appear in some public-transport applications, such as Google's
General Transit Feed SpecificationThe General Transit Feed Specification defines a common format for public transportation schedules and associated geographic information....
file format or some ticketing systems (e.g., in Copenhagen).
Digital clocks and watches using the 24-hour system usually show 00:00 at midnight. On some European brands of domestic appliance, such as ovens and microwaves, midnight is indicated by 24:00, continuing with 00:01.
Computer support
In most countries, computers by default show the time in 24-hour notation. For example,
Microsoft WindowsMicrosoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
activates the 12-hour notation by default only if a computer's language and region settings are:
- Albanian
- Chinese (Singapore and Taiwan)
- English (Australia, Belize, Canada, Caribbean, Jamaica, New Zealand, Philippines, Trinidad, South Africa, United States, and Zimbabwe)
- Greek
- Korean
- Spanish (Mexico and parts of South America)
- Swahili
Usually, users can easily switch to the 24-hour notation in such
localeIn computing, locale is a set of parameters that defines the user's language, country and any special variant preferences that the user wants to see in their user interface...
s, without affecting any of the other regional preferences.
Military time
In Canada and the United States, the term "military time" is a synonym for the 24-hour clock. In these regions, the time of day is customarily given almost exclusively using the
12-hour clockThe 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem and post meridiem...
notation, which counts the hours of the day as 12, 1, ..., 11 with suffixes "a.m." and "p.m." distinguishing the two
diurnalA day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
repetitions of this sequence. The 24-hour clock is commonly used there only in some specialist areas (military, aviation, navigation, tourism, meteorology, astronomy, computing, logistics, emergency services, hospitals), where the ambiguities of the 12-hour notation are deemed too inconvenient, cumbersome, or outright dangerous, with the military's use being the most famous example. The term "military time" has no particular meaning in most other regions of the world, where the 24-hour clock has long become a common element of everyday civilian life.
In the United States military, military time is similar to the 24-hour clock notation, with the exception that the colon is omitted and the time on the hours is often spoken as its decimal value. For instance, 6:00 a.m. would become 0600, and would be spoken "zero six hundred" or "zero six hundred hours" (for example, when said face-to-face), "oh six hundred" (colloquial and not strictly correct, as military communication protocols specify the word "zero" rather than "oh"), or "zero six zero zero" (for example, where clarity is needed when specifying the time over a radio or
sound-powered telephoneA sound-powered telephone is a communication device that allows users to talk to each other with the use of a handset, similar to a conventional telephone, but without the use of external power...
). However, none of these formatting or pronunciation details is exclusively military and all are common in the technical contexts in which the 24-hour clock is used in English-speaking countries.
Military usage differs in some respects from other twenty-four-hour time systems:
- Written military time does not usually include a time separator (for example, "0340" is more common than the civilian "03:40").
- Leading zero
A leading zero is any 0 digits, that lead a number string in a positional notation. For example, James Bond's famous identifier, 007, has two leading zeros. Leading zeros occupy most significant digits, which could be left blank or omitted for the same numeric value...
s, always written out by the military, are often also spoken in military usage, so 5:43 a.m. is often spoken "zero five forty-three" (military), as opposed to "five forty-three" (civilian).
- Military timestamp
A timestamp is a sequence of characters, denoting the date or time at which a certain event occurred. A timestamp is the time at which an event is recorded by a computer, not the time of the event itself...
s on messages have the time zoneA time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...
where applicable appended; in a civilian setting, this would be redundant. Military time zones are lettered and thus given word designations via the NATO phonetic alphabetThe NATO phonetic alphabet, more accurately known as the NATO spelling alphabet and also called the ICAO phonetic or spelling alphabet, the ITU phonetic alphabet, and the international radiotelephony spelling alphabet, is the most widely used spelling alphabet...
. For example, 6:00AM Eastern Standard TimeEastern Standard Time may refer to:*North American Eastern Time Zone, UTC-5*Australian Eastern Standard Time, UTC+10*An album by Hip Hop group Kooley High...
(GMT-5) would be written "0600R" and spoken, "zero six hundred Romeo".
- Local time is specifically designated in the military as zone J or "Juliet
The NATO phonetic alphabet, more accurately known as the NATO spelling alphabet and also called the ICAO phonetic or spelling alphabet, the ITU phonetic alphabet, and the international radiotelephony spelling alphabet, is the most widely used spelling alphabet...
". A time of "1200J" ("twelve hundred Juliet") corresponds to noon local time.
- Greenwich Mean Time
Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...
(or Coordinated Universal TimeCoordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
) is designated as zone Z or "Zulu". Usage of Z time is common in operations crossing multiple time zones.
History
The 24-hour time system has been used for centuries, primarily by scientists, astronomers, navigators, and horologists. There are many surviving examples of clocks built using the 24-hour system, including the famous Orloj in
PraguePrague is the capital and largest city of the Czech Republic. Situated in the north-west of the country on the Vltava river, the city is home to about 1.3 million people, while its metropolitan area is estimated to have a population of over 2.3 million...
, and the
Shepherd gate clockThe Shepherd Gate Clock is the clock mounted on the wall outside the gate of the Royal Greenwich Observatory building in Greenwich, London. The clock, an early example of an electric clock, was a slave mechanism controlled by electric pulses transmitted by a master clock inside the main building...
at
GreenwichGreenwich is a district of south London, England, located in the London Borough of Greenwich.Greenwich is best known for its maritime history and for giving its name to the Greenwich Meridian and Greenwich Mean Time...
.
At the
International Meridian ConferenceThe International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States to determine the Prime Meridian of the world. The conference was held at the request of U.S. President Chester A...
in 1884,
Sandford FlemingSir Sandford Fleming, was a Scottish-born Canadian engineer and inventor, proposed worldwide standard time zones, designed Canada's first postage stamp, a huge body of surveying and map making, engineering much of the Intercolonial Railway and the Canadian Pacific Railway, and was a founding...
proposed:
That this universal day is to be a mean solar day; is to begin for all the world at the moment of mean midnight of the initial meridian, coinciding with the beginning of the civil day and date of that meridian; and is to be counted from zero up to twenty-four hours.
This resolution was adopted by the conference.
According to a report in the London
TimesThe Times is a UK daily newspaper, the original English language newspaper titled "Times". Times may also refer to:In newspapers:*The Times , went defunct in 2005*The Times *The Times of Northwest Indiana...
in 1886, the 24-hour clock was in use on the
Canadian Pacific RailwayThe Canadian Pacific Railway , formerly also known as CP Rail between 1968 and 1996, is a historic Canadian Class I railway founded in 1881 and now operated by Canadian Pacific Railway Limited, which began operations as legal owner in a corporate restructuring in 2001...
train at Port Arthur.
The earliest country to introduce the 24-hour system nationally was Italy, in 1893. Other European countries followed: France adopted it in 1912 (the French army in 1909), followed by Denmark (1916), and Greece (1917). By 1920, Spain, Portugal, Belgium, and Switzerland had switched, followed by Turkey (1925), and Germany (1927). By the early 1920s, many countries in Latin America had also adopted the 24-hour clock. Some of the railways in India had switched before the outbreak of the war.
During
World War IWorld War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
, the British Royal Navy adopted the 24-hour clock in 1915, and the Allied armed forces followed soon after, with the British Army switching officially in 1918. The Canadian armed forces first started to use the 24-hour clock in late 1917. In 1920, the US Navy was the first US organization to adopt the system; the US Army, however, did not officially adopt the 24-hour clock until
World War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, on July 1 1942.
In Britain, the use of the 24-hour clock in daily life has grown steadily since the beginning of the 20th century, although attempts to make the system official failed more than once. In 1934, the
BBCThe British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
switched to the 24-hour clock for broadcast announcements and programme listings. The experiment was halted after five months following a lack of enthusiasm from the public, and the BBC has used the 12-hour clock ever since. In the same year, the US airlines
Pan American World Airways CorporationPan American World Airways, commonly known as Pan Am, was the principal and largest international air carrier in the United States from 1927 until its collapse on December 4, 1991...
and
Western AirlinesWestern Airlines was a large airline based in California, with operations throughout the Western United States, and hubs at Los Angeles International Airport, Salt Lake City International Airport, and the former Stapleton International Airport in Denver...
both adopted the 24-hour clock.
British RailBritish Railways , which from 1965 traded as British Rail, was the operator of most of the rail transport in Great Britain between 1948 and 1997. It was formed from the nationalisation of the "Big Four" British railway companies and lasted until the gradual privatisation of British Rail, in stages...
and
London TransportLondon Transport could refer to:*London Transport Transport authorities that operated services under the brand:*London Passenger Transport Board *London Transport Executive *London Transport Board...
switched to the 24-hour clock for timetables in 1964.
In 2005,
BBC WeatherBBC Weather is the BBC's department in charge of preparing and broadcasting weather forecasts and is now part of BBC News. The broadcast meteorologists are employed by the Met Office...
television forecasts used the 12-hour notation for several months after its graphical revamp. After complaints from the public, however, this was switched to 24-hour notation.
See also
- 24-hour analog dial
- Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
- Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
- Date and time representation by country
- Decimal time
Decimal time is the representation of the time of day using units which are decimally related. This term is often used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal...
- List of military time zones
- Six-hour clock
- Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
External links