| 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 The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem The 12-hour clock is a time...
|
| 00:00 |
12:00 a.m.
(start of day)
"12 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 p.m.
12 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 |
undefined
("12 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 equivalent to approximately 24 hours. It is not an SI unit but it is accepted for use with SI. The SI unit of time is the second....
runs from midnight to midnight and is divided into 24
hourThe hour is a unit of time. It is not an SI unit but is accepted for use with the SI.-Definition:In modern usage, an hour is a unit of measurement of time of the duration of 60 minutes, or 3600 seconds...
s, numbered 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 The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem The 12-hour clock is a time...
is, however, still dominant in a handful of countries, particularly the
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
,
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
(except
QuebecQuebec is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking identity and the only one whose sole official language is French at the provincial level....
) and
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
. 24-hour notation is also popularly referred to as
military timeIn Canada and the United States, the term military time is a synonym for the 24-hour clock notation, which counts the hours of the day from 00 to 23...
or
astronomical time in the
USThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
and
CanadaCanada is a country occupying most of northern North America, extending from the Atlantic Ocean in the east to the Pacific Ocean in the west and northward into the Arctic Ocean...
, and in
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
(though rarely). In some parts of the world, it is called
railway timeRailway time was the name given to the standardised time arrangement first applied by the Great Western Railway in England in November 1840. This was the first recorded occasion when a number of different local times were synchronised and a single standard time applied...
or
continental timeIn the United Kingdom, continental time refers to a time-keeping system used on the mainland of Europe, either the 24-hour clock, or the Central European Time zone. The former usage has become rarer as the UK uses the 24-hour clock more....
. It is also the international standard notation of time (
ISOThe International Organization for Standardization , widely known as ISO , is an international-standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization promulgates worldwide proprietary industrial and commercial...
8601ISO 8601 is an international standard covering the exchange of date and time-related data. The standard is titled “Data elements and interchange formats — Information interchange — Representation of dates and times” and was issued by the International Organization for Standardization...
). In the practice of
medicineMedicine is the art and science of healing. It encompasses a range 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 precludes any ambiguity as to what time events occurred in the 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, literally, "the middle of the night". In most systems it is when one day ends and the next begins: when the date changes. Originally midnight was halfway between sunset and dawn, 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, especially in computer applications, where many specifications require it (for example,
ISO 8601ISO 8601 is an international standard covering the exchange of date and time-related data. The standard is titled “Data elements and interchange formats — Information interchange — Representation of dates and times” and was issued by the International Organization for Standardization...
). 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:As with many other punctuation marks, the usage of colon varies among languages and, for a given language, among historical periods...
, which is also the symbol used in
ISO 8601ISO 8601 is an international standard covering the exchange of date and time-related data. The standard is titled “Data elements and interchange formats — Information interchange — Representation of dates and times” and was issued by the International Organization for Standardization...
. 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, literally, "the middle of the night". In most systems it is when one day ends and the next begins: when the date changes. Originally midnight was halfway between sunset and dawn, 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 some 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 avoiding ambiguity and does not represent a genuine clock time. 4th July 24:00 is, strictly speaking, 00:00 of the following date since it cannot be two dates as the same time.
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 guides 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 to report 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) are neither commonly used nor covered by any 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 and with time-validation stamps on some European public transport systems, such as those used in Copenhagen, which may show, for example, 27:45 instead of 03:45.
Most digital clocks and watches show times between 00:00 to 23:59. As a notable exception, the digital clocks of at least one European manufacturer of kitchen appliances show 24:00 for midnight, rather than the standard 00:00, with an example being Bosch Siemens microwaves.
The 24-hour clock in spoken English
- See also Names for the number 0
The U.S. military uses the 24-hour clock exclusively and would typically pronounce full hours as the number-word for the hour followed by "hundred" with an optional "hours" ("hours" is not used in the U.S. Navy or the U.S. Marine Corps) to clarify that the speaker is referring to a time of day. For instance, 1600 would be pronounced "sixteen hundred" or "sixteen hundred hours". The time 1830 is usually pronounced "eighteen thirty". 1000 is "ten hundred" rather than "one thousand"; hence, 2000 is "twenty hundred".
Both "eighteen o'clock" and "eighteen hundred" are commonly encountered spoken English for 18:00. Contrary to popular belief, the use of the word "hours" at the end of the stated time is not standard in the U.S. military. The use of the 24-hour clock without the word "hours" is the standard for expressing time in the U.S. military. The time 18:05 is commonly pronounced either "eighteen oh five" or "five past eighteen". In U.S. military usage, a leading zero for the hours before 10:00 is pronounced as well, as in "oh three oh five" or as "zero three zero five" for 03:05, but this would be considered unusual in a civilian setting.
Taking as an example, on many of the United Kingdom's railways, the public announcement system refers to 24-hour times as: 06:59 "oh six fifty-nine", to 07:00 "oh seven hundred hours". "Midnight exactly" is used for 00:00 (however no train in the United Kingdom is shown to depart at 0000 or 2400 to avoid any ambiguity), but when they depart at, for example, 00:26, it is announced as "midnight twenty-six". It is also common to hear the hour spoken as "seven hours Greenwich Mean Time" (written 07:00, for instance), as heard on the BBC World Service radio broadcasts in the US. On the display boards at Birmingham New Street, mirroring the majority of stations in the UK, and timetables, the time is written as HHMM, as in 0659 or 0700 for 'one minute to-' and 'seven o'clock' respectively.
In common with what happens with units, the written and spoken forms of time do not always match. For example, it is possible for a train time to be written as "18:30" but a person may say "there is a train at half-past".
Computer support
In most countries, computers by default show the time in 24-hour notation. For example,
Windows XPWindows XP is a line of operating systems produced by Microsoft for use on personal computers, including home and business desktops, laptops, and media centers. The name "XP" is short for "eXPerience"...
activates the 12-hour notation by default only if a computer's language and region settings are:
- Albanian
- English (only in Australia, Belize, Canada, Caribbean, Jamaica, New Zealand, Philippines, Trinidad, South Africa, United States, and Zimbabwe)
- Greek
- Spanish (only in 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.
History
The 24-hour time system has been used for centuries, primarily by scientists, astronomers, navigators, and horologists. Numerous examples of clocks using the 24-hour system have been built, ranging from
John HarrisonJohn Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought and critically-needed key piece in solving the problem of accurately establishing the East-West position, or longitude, of a ship at sea, thus revolutionising and extending the possibility of...
's H1 chronometer to 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 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.The town became the site of a Royal palace, the...
.
The British and Canadian militaries first started to use the 24-hour clock in late 1917. Previous to this, military orders were drafted using the familiar "a.m." and "p.m." suffixes.
In Britain, the use of the 24-hour clock in daily life has grown steadily since the beginning of the 20th century. In 1934, the
Radio TimesRadio Times is the BBC's weekly television and radio programme listings magazine. It also provides on-line listings.-History and publication:...
switched to the 24-hour clock for programme listings. The experiment was halted after five months following widespread protests from readers, and has used the 12-hour clock ever since. More recently,
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 in 2005, after its graphical revamp. After complaints from the public, however, this was switched to 24-hour notation.
See also
- Time
Time is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects...
- Clock
A clock is an instrument used to indicate, measure, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell"...
- 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...
- ISO 8601
ISO 8601 is an international standard covering the exchange of date and time-related data. The standard is titled “Data elements and interchange formats — Information interchange — Representation of dates and times” and was issued by the International Organization for Standardization...
- Zulu time
- 24 hour analog dial
Clocks and watches with a 24-hour analog dial have an hour hand that makes one complete revolution, 360°, in a day...
- 12-hour clock
The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods called ante meridiem The 12-hour clock is a time...
- Thai 6-hour clock
The six-hour clock is a traditional timekeeping system used in Thailand alongside the official twenty-four-hour clock. Like the other common systems, it counts twenty-four hours in a day, but divides the day into four quarters, counting six hours in each. The hours in each quarter are told with...
- Date and time notation by country
Different style conventions and habits exist around the world for dates and times in writing and speaking. Examples:*The order that a year, month, and day are written.*How weeks are identified.*The 24-hour clock and/or the 12-hour clock....
External links