|24-hour clock||12-hour clock|
|00:00|| 12:00 a.m.|
(start of day)
|12:00|| 12:00 p.m.|
|24:00|| 12 midnight|
(end of day)
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 today. The 12-hour clock is, however, still dominant in a handful of countries, particularly the United States and Canada (except Quebec). 24-hour notation is also popularly referred to as military time or astronomical time in the US and Canada, and in Australia (though rarely) as army time. In some parts of the world, it is called railway time. It is also the international standard notation of time (ISO 8601).
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 decimal number of full hours that have passed since midnight, 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 second, 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 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. 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. In some contexts (e.g., U.S. military, some computer protocols), no separator is used (e.g., 2359) and in some countries (e.g. France) the letter h is sometimes used when indicating hours and minutes only (18h45).
In the 24-hour time notation, the day begins at midnight, 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.
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.
Practically all models of digital wristwatches and clocks available outside the United States display the time of day using the 24-hour notation by default. 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 insufficient by many customers outside the United States or Canada.
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".
Usually, users can easily switch to the 24-hour notation in such locales, without affecting any of the other regional preferences.
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, although it has not always been enthusiastically adopted by the general public. In 1934, the Radio Times 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.