24-hour clock /ˌtwenti fɔːr aʊə ˈklɒk/$/ˌtwenti fɔːr aʊər ˈklɑːk/

the system of using twenty-four numbers to talk about time

24-hour clock system is a method of designating time by using the numeric sequence from 00 to 23 for the hours and the numbers 00 to 59 for the minutes in a daily cycle beginning with 0000 (midnight) and ending with 2359 (1 minute before the next midnight). The system provides a clear distinction between pre-noon and afternoon time without requiring the designations AM and PM.

But why clock? Why not watch or time or system?

According to Merriam-Webster, “clock” is a device other than a watch for indicating or measuring time commonly by means of hands moving on a dial; broadly : any periodic system by which time is measured.

This last part is what is meant in the phrase “24-hour clock”: clock: any periodic system by which time is measured.


“Hours” [plural] is used when giving the time according to the 24-hour clock, usually in military or other official language:

“The first missile was launched at 2300 hours (= at 11 p.m.).” This is pronounced ‘23 hundred hours’. “The helicopters lifted off at 0600 hour.” “1530 hours” “1805 hours”

Using “hours” is telling the time reckoned in one 24-hour period from midnight to midnight with a 4-digit number of which the first two digits indicate the hour and the last two digits indicate the minute:

“In the military 4:30 p.m. is called 1630 hours.” ”By 1300 hours the position was fairly clear.” “The first bomb fell at 0051 hours.” “The monthly charge covers scheduled servicing carried out between 08:00 to 18:00 hours Monday to Friday.” “Visa applications can be submitted from Monday to Friday from 08:00 hours to 12:00 hours and from 13:00 to 16:00 hours.”

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 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. A leading zero is added for numbers under 10, but it is optional for the hours.

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. In some contexts (including the U.S. military and some computer protocols), no separator is used and times are written as, for example, “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 one day is the same 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 until the end date at 24:00.

While the 24-hour notation unambiguously distinguishes between midnight at the start (00:00) and end (24:00) of any given date, there is no commonly accepted distinction among users of the 12-hour notation.

The 24-hour clock is commonly used in some specialist areas (military, aviation, navigation, tourism, meteorology, astronomy, computing, logistics, emergency services, hospitals), where the ambiguities of the 12-hour notation are deemed too inconvenient, cumbersome, or dangerous.

Military usage, as agreed between the United States and allied English-speaking military forces, differs in some respects from other twenty-four-hour time systems:

  • No hours/minutes separator is used when writing the time, and a letter designating the time zone is appended (for example “0340Z”).
  • Leading zeros are always written out and are required to be spoken, so 5:43 a.m. is spoken “zero five forty-three” (casually) or “zero five four three” (military radio), as opposed to “five forty-three” or “five four three”.
  • Military time zones are lettered and thus given word designations via the NATO phonetic alphabet. For example, 6:00 a.m. US Eastern Standard Time (UTC−5) would be written “0600R” and spoken “zero six hundred Romeo”.
  • Local time is designated as zone J or “Juliett”. “1200J” (“twelve hundred Juliett”) is noon local time.
  • Greenwich Mean Time (or Coordinated Universal Time) is designated time zone Z, and thus called “Zulu time”.
  • Hours are always “hundred”, never “thousand”; 1000 is “ten hundred” not “one thousand”; 2000 is “twenty hundred” not “two thousand”.

  • Research Questions:
  1. What other types/methods of telling time are there in the English language and in the world? Which ones are more formal and which one are more informal?
  2. What is 12AM and 12PM? Which one is noon (12:00) and which is midnight (24:00)?
  3. Why are there 24 hours a day? Why not more and why not less? What is the history of 24-hour clock?