| 24-hour clock | 12-hour clock |
|---|---|
| 00:00 | 12:00 midnight |
| 01:00 | 1:00 am |
| 02:00 | 2:00 am |
| 03:00 | 3:00 am |
| 04:00 | 4:00 am |
| 05:00 | 5:00 am |
| 06:00 | 6:00 am |
| 07:00 | 7:00 am |
| 08:00 | 8:00 am |
| 09:00 | 9:00 am |
| 10:00 | 10:00 am |
| 11:00 | 11:00 am |
| 12:00 | 12:00 noon |
| 13:00 | 1:00 pm |
| 14:00 | 2:00 pm |
| 15:00 | 3:00 pm |
| 16:00 | 4:00 pm |
| 17:00 | 5:00 pm |
| 18:00 | 6:00 pm |
| 19:00 | 7:00 pm |
| 20:00 | 8:00 pm |
| 21:00 | 9:00 pm |
| 22:00 | 10:00 pm |
| 23:00 | 11:00 pm |
| 24:00 | 12:00 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 (and 24 in the day-ending midnight). This system is the most commonly used time notation in the world of 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 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 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 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 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 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 most well 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
The 24-hour notation is very commonly used in the United Kingdom. It is almost the only format used on digital clocks, time tables and with computers. Counterexamples are usually due to imported U.S. technology. The 24-hour notation is commonly used in written communication, but its use there is not yet quite as universal as in much of the non-English speaking world. The 12-hour notation is still used occasionally in written communication and displays, especially by elderly people, and remains popular in informal spoken language.
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 talking casually.
It is not uncommon that the same person would use the 24-hour notation in spoken language when refering to an exact point in time ("The train leaves at fourteen fourty-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.
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.