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In American English, the term military time is a synonym for the 24-hour clock. [8] In the US, the time of day is customarily given almost exclusively using the 12-hour clock notation, which counts the hours of the day as 12, 1, ..., 11 with suffixes a.m. and p.m. distinguishing the two diurnal repetitions of this sequence.
The 24-hour clock is used in military, public safety, and scientific contexts in the United States. [4] It is best known for its use by the military and is therefore commonly called "military time". In U.S. military use, 24-hour time is traditionally written without a colon (1800 instead of 18:00).
The term is less commonly used for the time display on digital clocks and watches. A second type of clock face is the 24-hour analog dial, widely used in military and other organizations that use 24-hour time. This is similar to the 12-hour dial above, except it has hours numbered 1–24 (or 0–23) around the outside, and the hour hand makes ...
John Harrison, Thomas Tompion, and Mudge [7] built a number of clocks with 24-hour analog dials, particularly when building astronomical and nautical instruments. 24-hour dials were also used on sidereal clocks. The famous Big Ben clock in London has a 24-hour dial as part of the mechanism, although it is not visible from the outside. [8]
Times relative to the designation are indicated with +/−[Arabic numeral] after the letter, replacing -day or -hour with a count of the same unit: "D−1" (the day before D-Day), "L+9" (9 hours after L-Hour) etc. [citation needed] In less formal contexts, the symbol or numeral may be spelled out: "D minus 1" or "L plus nine."
RFC 733 published in 1977 allowed using military time zones in the Date: field of emails. [11] RFC 1233 in 1989 noted that the signs of the offsets were specified as opposite the common convention (e.g. A=UTC−1 instead of A=UTC+1), [ 12 ] and the use of military time zones in emails was deprecated in RFC 2822 in 2001.
Even in those countries where the 12-hour clock is predominant, there are frequently contexts (such as science, medicine, the military or transport) in which the 24-hour clock is preferred. In most countries, however, the 24-hour clock is the standard system used, especially in writing. Some nations in Europe and Latin America use a combination ...
Midnight to 1 a.m. on a 24-hour clock with a digital face. An hour (symbol: h; [1] also abbreviated hr) is a unit of time historically reckoned as 1 ⁄ 24 of a day and defined contemporarily as exactly 3,600 seconds . There are 60 minutes in an hour, and 24 hours in a day.