24-hour clock

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

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).

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". 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.

Digital clocks run from 00:00 to 23:59, that is 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.

File:Railtime.png

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:

There is no possibility of ambiguity between times in the morning and evening (in the 12-hour system "seven o'clock" can mean both 7 am and 7 pm). In reading schedules and the like, it is easy to see at a glance whether times refer to before or after noon. This is especially important for organizations that run services 24 hours a day, such as airlines, railways, and the military.
Displays that use the 12-hour system usually show noon as 12:00 pm and midnight as 12:00 am — a convention which is ambiguous and therefore confuses many people. The workaround of writing "noon" or "12 midnight" requires more space, makes the notation language dependent, and still fails to distinguish between midnight at the start and at the end of a day. Such problems have led in the United States to the practice of avoiding deadlines at noon or midnight entirely. In the 24-hour notation there are no such problems. Midnight at the start of a day is simply 00:00, noon is 12:00, and midnight at the end of a day is 24:00.

The duration of time intervals is easier to see in the 24-hour notation. From 10:30 a.m. till 3:30 p.m. is 5 hours. From 10:30 till 15:30 indicates this more clearly.

The 24-hour notation is shorter, which can save space in tables.

The 12-hour notation obscures the fact that the date changes between 11:59 p.m. and 12:00 a.m., which regularly confuses people who program their video recorder. The transition from 23:59 to 00:00, on the other hand, provides a clear reminder that a new date starts.

Helps avoid mistakenly setting digital or analog alarm clocks for the "p.m." instead of the "a.m." hour, or vice versa.

The 24-hour notation (when used with leading zero) is sorted correctly automatically by alphabetical comparison functions in computer programs, for example:

 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:

Most analog clocks have dials of 12 hours; therefore, the user has to associate two numbers for each position of the hour hand.

Some people find the idea of 0 or 00 hours unnatural, even though the notion of 00 minutes is commonly accepted.

Clocks with a chime are in step only on hours 1 to 12, and deviate for hours 0, and 13 to 23.

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.

Non-English speaking world

The 24-hour clock enjoys broad everyday usage in most Asian, 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.

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

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:

Albanian
English (United States, Canada, South Africa, Zimbabwe, Australia, New Zealand, Philippines, Belize, Trinidad, Jamaica, Caribbean)
Greek
Spanish (Mexico, parts of South America)
Swahili

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

On Microsoft Windows, open the Control Panel, select Regional and Language Options, then go to the Regional Options tab and press Customize…. In the newly opened window select the Time tab and choose in the Time format: field the option HH:mm:ss. (For the full ISO 8601 experience, also go to the Date tab and choose in the Short date format: field the yyyy-MM-dd option.)
If using Mac OS X, open System Preferences, then choose Date & Time, then select the Clock tab, select Use a 24-hour clock and deselect Show AM/PM.