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24-hour clock

Encyclopedia : 2 : 24 : 24H : 24-hour clock

24-hour clock 12-hour clock
00:00 12:00 a.m. (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. (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 12:00 a.m. (midnight)

The 24-hour clock is a convention of time-keeping in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23. This system is the most commonly used time notation in the world of today. The United States and Canada are the only industrialized countries left in which a substantial fraction of the population is not yet accustomed to it. The 24-hour notation is in the US and Canada also referred to as military time, and (now only rarely) in the United Kingdom as continental time. It is also the international standard notation of time (ISO 8601).

Contents

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 (00–23) is the decimal number of full hours that have passed since midnight, mm (00–59) is the number of full minutes that have passed since the last full hour, and ss (00–59) is the number of seconds since the last full minute. 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 8601). 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. In the case of a leap second, the value of ss may extend to 60. The most commonly used separator symbol between hours, minutes and seconds is the colon, 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.

Midnight 00:00 and 24:00

In the 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day is that beginning at 23:59. Where convenient, the notation 24:00 may be used in addition to refer to midnight at the end of a given date, that is 24:00 of the current day is the same point in time as 00:00 of the following day.

The notation 24:00 mainly serves to clarify that 00:00 unambiguously marks the midnight associated with the start of a date, but it can also be useful for referring to the exact end of a day in a time interval. A typical usage are opening hours till midnight, e.g. "00:00–24:00", "07:00–24:00". Furthermore, conventional railway timetables show 00:00 as depature time and 24:00 as arrival time.

Time-of-day notations beyond 24:00 (such as 24:01 or 25:59) are neither commonly used nor covered by any relevant standards, although they have been seen on rare occasions in Japan. However, in producing television programmes in Hong Kong, especially for drama series or situation comedies (which often require the crew and the cast to work until very late night), such notations beyond 24:00 are commonly used orally. An example is "I (have got to) work till 28 tonight." (i.e. 04:00 on the next day)

Digital clocks run from 00:00 to 23:59, this means they never show 24:00 on their display. This way, the roll-over from 23:59:59.999 to 00:00:00.000 coincides with the start of a new day and date.

Example of a railway timetable that shows both 00:00 (for departure times) and 24:00 (for arrival times).
Enlarge
Example of a railway timetable that shows both 00:00 (for departure times) and 24:00 (for arrival times).

Relation to the 12-hour notation

The 12-hour and 24-hour notations look similar from 1:00 a.m. to 12:59 p.m. (01:00 to 12:59), except that the 24-hour notation has no a.m./p.m. suffix. To convert a 12-hour time to the 24-hour notation, from 1:00 p.m. to 12 midnight (13:00 to 24:00), one has to add 12 h, and from 12 midnight to 12:59 a.m. (00:00 to 00:59) one has to subtract 12 h. See also the table to the right.

Practically all models of digital wristwatches and clocks available outside the United States display the time of day by default using the 24-hour notation. Most can also be switched into a 12-hour mode, for U.S. customers. Equipment that supports only the 12-hour notation is likely to be considered deficient in functionality by many customers outside the United States, the United Kingdom or Canada.

Advantages

The 24-hour notation has many advantages over the 12-hour system:

00:30     <  11:30     <  12:30     <  23:30     timesort
"00:30"    < "11:30"    < "12:30"    < "23:30"    stringsort (same result)
12.30 a.m.  <  11.30 a.m.  <  12.30 p.m.  <  11.30 p.m.  timesort
"11.30 a.m." < "11.30 p.m." < "12.30 a.m." < "12.30 p.m." stringsort (different result)

Disadvantages

Some disadvantages to the 24 hour system that have been suggested might be:

Use by country

Main article: date and time notation by country

United States

The United States differs from other countries in that a significant fraction of its population may not yet be familiar with the 24-hour time notation. The 12-hour notation is the by far dominant time notation in the U.S., and the 24-hour notation is rarely used so far in public communication. The 24-hour notation is best known in the U.S. for its use by the military, where it is traditionally written without a colon (1800 instead of 18:00) and in spoken language followed by the word "hours". It is also widely used by astronomers and some other communities (public safety, transport, aerospace) where exact and unambiguous communication of time is critical. It is also widely used with computers, but less commonly with "user-friendly" computer applications targeted at non-specialist end users. Even airline tickets use the 12-hour notation in the United States.

United Kingdom

Both the 12-hour and 24-hour notations are used in the United Kingdom. The 12-hour notation is still widely used in ordinary life, written communication and displays and continues to be used in informal spoken language. The 24-hour notation is used in timetables and some written communication, but its use there is not universal as in much of the non-English speaking world.

Canada

In Canada, similar to the United States and the United Kingdom, the 12-hour clock is used in ordinary life. An exception to this, however, is in Quebec, where French speakers often use the 24-hour clock. 24-hour time is also very common in the capital, Ottawa, Ontario.

Australia

Australia is not dissimilar to the United States. The 12-hour notation is the standard, used in television schedules, timetables and other public communications; 24-hour notation is limited to internal communications within services and organisations. 24-hour notation is usually referred to as "army time", mainly by those who fail to grasp the 24-hour concept. Some Australians even react angrily to 24-hour notation, scolding others for presenting something confusing. (Occasionally, when confronted with 24-hour notation, they will still ask, "Is that in the morning or the evening?") It would appear that Australia will never migrate to 24-hour notation.

Other regions

The 24-hour clock enjoys broad everyday usage in most African, Asian, Oceanic, European and many Latin American countries. When a time is written down or displayed, the 24-hour notation is used in these countries almost exclusively. The 12-hour clock remains in some regions commonly used in informal language, while, for example, most German, French and Romanian speakers use the 24-hour clock today even when speaking casually.

It is not uncommon that the same person would use the 24-hour notation in spoken language when referring to an exact point in time ("The train leaves at fourteen forty-five …"), while using some variant of the 12-hour notation to refer vaguely to a time ("… so I will be back tonight sometime after five."). People are used to converting between the two notations without requiring mental arithmetic, and most perceive "three o'clock" and "15:00" simply as synonyms.

In many East African languages, the start of the daily time system is at dawn, not midnight. Thus what would be seven o'clock in the morning in English becomes one o'clock in the morning in Swahili and other East African languages. This also affects the date: the whole night is the same date as the preceding day. For example, Tuesday does not become Wednesday until morning breaks, rather than changing at midnight. For multi-lingual speakers in East Africa, the convention is to use the time system applicable to the language one happens to be speaking at the time. A person speaking of an early morning event, in Engilish, would report that it happened at eight o'clock, however in repeating the same facts in Swahili, would state that the events occurred at "saa mbili" (two o'clock). The 24-hour notation does not have application in East African languages.

The 24-hour clock in spoken English

The time 18:30 is usually pronounced "eighteen thirty". In U.S. military usage, this is often followed by the word "hours", to clarify that the speaker is referring to a time of day. Conventions differ slightly for full hours, but both "eighteen o'clock" and "eighteen hundred" are commonly encountered spoken English for 18:00, with "eighteen hundred hours" being the standard U.S. military usage. 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 hours" 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". What is used for 00:00 is unknown, as trains virtually never leave at midnight, but when they depart at, for example, 00:26, it is announced as "midnight twenty-six". 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

A rare example of a digital clock showing minutes 00:01–24:00 (rather than the standard 00:00–23:59)
Enlarge
A rare example of a digital clock showing minutes 00:01–24:00 (rather than the standard 00:00–23:59)

In most countries, computers by default show the time in 24-hour notation. The 12-hour notation is typically set by default if a computer's language and region settings are:

Usually, users can easily switch to the 24-hour notation in such locales, without affecting any of the other regional preferences:

See also

 

From Wikipedia, the Free Encyclopedia. Original article here. Support Wikipedia by contributing or donating.
All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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