The 24-hour clock counts the hours of the day from 00 to 23 instead of splitting them into two twelve-hour runs. Midnight is 00:00, noon is 12:00, and 11 at night is 23:00. Because every time of day has exactly one label, there is never any doubt about whether "7 o'clock" means morning or evening.
In the United States the format is often called military time, because the armed forces use it exclusively. Elsewhere it is simply the normal way to write the time: train timetables in France, hospital charts in Germany and shop opening hours in Japan all use the 24-hour clock.
How to read it
For any hour from 00:00 to 11:59, the 24-hour time is the morning time you already know, with 00:00 being midnight. For hours from 13:00 onward, subtract twelve to get the afternoon or evening time: 14:00 is 2 PM, 18:30 is 6:30 PM, 21:45 is 9:45 PM. With a little practice, "twenty hundred hours" starts to feel as natural as "eight PM".
- 00:00 — midnight, the start of the day.
- 06:00 — 6 AM, a typical sunrise hour.
- 12:00 — noon.
- 18:00 — 6 PM.
- 23:59 — one minute before the next day begins.
How the military speaks it
Armed forces and aviation add a spoken convention: times are read digit by digit, always with four digits. 08:00 is "zero eight hundred hours", 15:30 is "fifteen thirty hours", and 00:00 is "zero hundred hours" or simply "midnight". When a time zone must be pinned down, a letter suffix is added — Z ("Zulu") for UTC, so 14:00 UTC is "fourteen hundred Zulu". This removes the last possible ambiguity: not just which half of the day, but which part of the planet the time refers to.
Why precision fields prefer it
Anywhere a misunderstanding about AM and PM could cost money or safety, the 24-hour clock is standard. Pilots, air traffic controllers, surgeons, railway dispatchers and software systems all use it. A medication order for "7:00" is unambiguous; "7 o'clock" is not. The same logic applies to logs and databases, which is why computers internally keep time as a running count described in our Unix timestamp guide.
If you would like to see the 24-hour format in action, our world clock follows your device's own preference, and the time zone converter accepts 24-hour input directly. For the meaning of AM and PM themselves, see what AM and PM actually mean.