What clock format writes times from 00:00 to 23:59?

The 24-hour clock writes times from 00:00 through 23:59.

In this system, the day begins at midnight, written 00:00, and the hours run from 00 to 23. Minutes run from 00 to 59, so 18:30 means 6:30 p.m. in the 12-hour system. Seconds can be added in a form such as 18:30:45.

The format avoids the a.m./p.m. ambiguity of the 12-hour clock. It is common in transport timetables, hospitals, computing, military communication, science, and international standards. Leading zeroes are especially common in technical writing, although some contexts allow 0:00 rather than 00:00.

A common point of confusion is midnight. 00:00 means the start of a date, while ISO 8601 can also use 24:00 to mean the end of that date—the same instant as 00:00 on the following day. The 24-hour clock is a time-notation system, not sidereal time, which tracks Earth’s rotation relative to distant stars rather than the ordinary solar day.