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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 (except Quebec). 24-hour notation is in the US and Canada also referred to as military time or astronomical time,^[1] 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).

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 the current day is the same point in 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.

Digital clocks and watches usually show times between 00:00 to 23:59. The roll-over of the hour indicator from 23 to 00 coincides with the start of a new day and date. (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.)

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

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), 12 hours are added, and from 12 midnight to 12:59 a.m. (00:00 to 00:59) 12 hours are subtracted. See also the table to the above.

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.

Main article: Comparison of the 12-hour and 24-hour clocks

Main article: date and time notation by country

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. 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"^{[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".

A rare example of a digital clock showing midnight as 24:00 instead of the standard 00:00. This variant notation has been used by at least one European supplier of kitchen appliances.

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.

• Time
• Clock
• Boundaries of the day
• Decimal time
• ISO 8601
• Zulu time
• 24 hour analog dial
• 12-hour clock
• Thai 6-hour clock

• The 24 hour clock