24-hour clock

The 24-hour clock, also referred to as military time (only in US) or continental time, is a convention of time keeping where the time of day is reported in one continuous cycle of twenty-four hours, unlike the 12-hour clock with its one cycle of twelve A.M. hours followed by another of twelve P.M. hours.

Under the 24-hour clock system, the calendar day begins at the stroke of midnight, 00:00 (the same as 12:00 A.M.), and ends at 23:59:59 (11:59:59 P.M.). For convenience however it is permissible to say that something ending at midnight, like a late shift or a store closing, ends at 24:00 (which is equivalent to 00:00 the next day). Note that in US military time, the first colon is omitted, so 21:00 would become 2100 and read as "twenty-one hundred hours".

One of the main aims of the 24-hour clock is to avoid ambiguity between A.M. and P.M. times in fields of endeavour which can occur at any time of day or night, such as civil aviation, railways, or military maneuvers.

Illustration: If someone were to leave a message asking you to meet with them at seven o'clock the next day and go eat without specifying A.M. or P.M. or even what meal, you would have to ask them at least which part of the day they meant. A nice thing about the 24-hour clock is that your friend would have to tell you either 07:00, for breakfast, or 19:00, for dinner.

Conversion to or from continental time is done as follows.

To convert to the 24-hour clock, add 12 to the hours if the time is between 1:00 P.M. and 11:59 P.M., otherwise leave the time as is, except for the hour 12:00-12:59 A.M., which becomes 00:00-00:59. Thus, 12:16 A.M. becomes 00:16, 10:14 A.M. becomes 10:14, and 5:46 P.M. becomes 17:46.

To convert from the 24-hour clock, subtract 12 hours from times between 13:00 and 23:59, which are P.M., otherwise do nothing, except for the hour 00:00-00:59, which becomes 12:00-12:59 A.M. Thus, 19:23 becomes 7:23 P.M., 12:32 becomes 12:32 P.M., and 06:51 becomes 6:51 A.M.

As well as military, public safety, and transport applications, the 24-hour clock enjoys also broad everyday civilian usage in many European and Latin American countries, and is widely used in establishing settings for computer operating systems.

Many models of digital wristwatches and some clocks are available that will display the time of day in a 24-hour format.

See also:

• Zulu time
• fencepost error