24-hour clock

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24-hour clock 12-hour clock
00:00 12 midnight
(start of day)
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 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 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 dominant instead in a handful of countries, particularly the United States and Canada. 24-hour notation is in the US and Canada also referred to as military time, and in Australia 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).

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[edit] 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 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 many contexts, especially with a time zone, no seperator is used hence 1.43 p.m. in Greenwich in winter becomes 1343 GMT.

[edit] 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 is giving opening hours ending at midnight, eg "00:00–24:00", "07:00–24:00". Similarly, some railway timetables show 00:00 as departure 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. However, they have been observed occasionally in some special contexts in Japan and Hongkong where business hours extend beyond midnight, such as broadcast-television production.

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

[edit] 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 11:59 p.m. (13:00 to 23:59), one has to add 12 hours, and from 12 midnight to 12:59 a.m. (00:00 to 00:59) one has to subtract 12 hours. 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 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.

[edit] Advantages

Suggested advantages to the 24 hour system, compared to the 12-hour system, include:

  • 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 a.m. and 7 p.m.). In reading schedules and the like, it may be easier 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 p.m. and midnight as 12:00 a.m. — a convention which can be confusing. Writing "noon" or "12 midnight" requires more space, makes the notation language dependent, and does not allow for the 24-hour practice of associating midnight with either the ending or beginning day. Such problems have led in the United States to the practice of avoiding deadlines at midnight entirely.
  • 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 can lead to confusion. The transition from 23:59 to 00:00, on the other hand, provides a clear reminder that a new date starts.
  • The 24-hour notation 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 24-hour system
"00:30"      < "11:30"      < "12:30"      < "23:30"      stringsort 24-hour system (same result)
 12:30 a.m.  <  11:30 a.m.  <  12:30 p.m.  <  11:30 p.m.  timesort 12-hour system
"11:30 a.m." < "11:30 p.m." < "12:30 a.m." < "12:30 p.m." stringsort 12-hour system (different result)

[edit] Disadvantages

Main article: 12-hour clock#Advantages over the 24-hour clock

Suggested disadvantages to the 24 hour system include:

  • Most analog clocks and watches have dials of 12 hours; therefore, the user has to associate two numbers for each position of the hour hand.
  • 24 hours analog dials have smaller spaces between hours, and so some people find it harder to read the time. However, this isn't primarily a problem related to the 24-hour clock system: many 24 hour analog dials, particularly from the medieval and renaissance period, use the 12-hour clock, two sets of 12 hours displayed on the same dial, and might cause some people similar difficulties.
  • 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.
  • In written English, there is no conventional notation for indicating that a time uses the 24-hour clock (other than the absence of an am/pm suffix). Therefore, a 24-hour time between 1:00 and 12:59 might be considered ambiguous by some readers.
  • In terms of pronunciation, a specific 24-hour notation time such as "19:59", said as "nineteen fifty-nine", may sound too similar to and be mistaken as the calendar year 1959. Though this issue has been nonexistent since 1960, it may resurface after 2010, when the pronunciation "twenty-ten" (20-10) may become more prevalent than "two-thousand-ten" (2,010).

[edit] Use by country

Main article: date and time notation by country

[edit] The 24-hour clock in spoken English

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" to clarify that the speaker is referring to a time of day. For instance, 16:00 would be pronounced "sixteen hundred" or "sixteen hundred hours". The time 18:30 is usually pronounced "eighteen thirty".

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" or as "zero three zero 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"[citation needed]. "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".

[edit] Computer support

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

  • 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 locales, without affecting any of the other regional preferences.

[edit] See also

[edit] References

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