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 also referred to (only in the US and Canada) as military time or (only in the United Kingdom and now very rarely) as continental time. It is the international (ISO) standard notation of time. A time in the 24-hour clock is written in the form hours:minutes (for example, 01:23), or hours:minutes:seconds (01:23:45), where a leading zero is used for numbers under 10. Under the 24-hour clock system, 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, which is identical to 00:00 of the following day.

To convert between the 12-hour clock and the 24-hour clock, use the table to the right.

As well as military, public safety, and transport applications, the 24-hour clock enjoys broad everyday civilian usage in many Asian, European and Latin American countries, where it is almost exclusively used in written times. It is also widely used in establishing settings for computer operating systems. The vast majority of the French and Romanian population use this system even when talking casually.

Many models of digital wristwatches and clocks are available that display the time of day using the 24-hour clock.

The notation 24:00 is used for example in 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".

The 24-hour clock has a number of 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), and 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.
• The 12-hour clock often writes noon as 12 pm and midnight as 12 am: a convention which is ambiguous and confuses many people. The 24-hour clock notates midnight at the beginning of the day as 00:00, noon as 12:00, and midnight at the end of the day as 24:00.
• Duration of time intervals is easier to see. From 11:30 am till 12:30 am is 13 hours (not one as it may seem). From 11:30 till 00:30 indicates this better.
• The date of times just after midnight (12:01 am is on the next day of 11:59 pm two minutes before) is less easily mistaken, as many people have found out to their detriment when programming a video recorder (transition from 23:59 to 00:01 shows this clearer).

See also

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