24-hour clock

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 world-wide the most commonly used time notation today. The United States is the only industrialized country 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 refered 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 in the 24-hour notation is written in the form hours:minutes (for example, 01:23), or hours:minutes:seconds (01:23:45). 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 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day is that beginning at 23:59 and ending at 24:00. The time 24:00 of the current day is identical to 00:00 of the following day. 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. However, the notation 24:00 is useful for refering to the exact end of a day in a time interval.

The 12-hour and 24-hour notations look similar from 1:00 am to 12:59 pm (01:00 to 12:59), except that the 24-hour notation has no am/pm suffix. From 1:00 pm to 12 midnight (13:00 to 24:00), one has to add 12 h to convert a 12-hour time to the 24-hour notation, and from 12 midnight to 12:59 am (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 only supports the 12-hour notation is likely to be considered deficient in functionality by many customers outside the United States.

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" means 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 "12 noon" or "12 midnight" requires more space, makes the notation language dependant, and still fails to distinguish between midnight at the start and at the end of a day. Such problems have lead 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 am till 3:30 pm is 5 hours. From 10:30 till 15:30 indicates this better.
• The 24-hour notation is shorter, which can save precious space in tables.
• The 24-hour notation (when used with leading zero) is sorted correctly automatically by alphabetical comparison functions in computer programs, for example "11:00" < "22:00", whereas this fails under the 12-hour notation in "10:00 pm" < "11:00 am".
• The 12-hour notation obscures the fact that the date changes between 11:59 pm and 12:00 am, 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.

The notation 24:00 is used, for example, in many railway timetables, to indicate the end of the day. Thus a train due to arrive at a station during the last minute of any particular day may be shown as doing so at 24:00; trains due to depart during the first minute of the day are shown as leaving at 00:00. It is also practiced for opening hours till midnight, e.g. "00:00–24:00", "07:00–24:00".

Use by country

United States

The 24-hour notation is most well known 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 unambiguous communication of time is critical. It is also widely used in establishing settings for computer operating systems.

United Kingdom

While the 24-hour notation is not yet as universally used as in much of the the non-English speaking world, it has long become the most commonly used notation on time tables and computers.

Non-English speaking world

The 24-hour clock enjoys broad everyday civilian usage in most Asian, European and Latin American countries, where it is almost exclusively used to write down times. The vast majority of German, French and Romanian speakers use this system even when talking casually.

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". Conventions differ slightly for full hours, but both "eighteen o'clock" and "eighteen-hundred" are commonly encountered spoken English for 18:00. In U.S. military usage, the leading zero is pronounced as well, as in "oh three oh five hours" for 03:05, but this would be considered unusual in a less formal civilian setting.

See also

• 12-hour clock
• ISO 8601
• Zulu time
• fencepost error
• Clock
• Decimal time
• Boundaries of the day