Which format denotes a 24 hour clock?

The format that denotes a 24-hour clock is represented by HH24. In this notation, hours are displayed in a range from 00 to 23, which corresponds to the full 24-hour day. For example, "00:00" represents midnight, while "23:00" signifies eleven o'clock at night. This format is widely used in various applications and systems where clarity and precision are required, particularly in international contexts where the 12-hour format might cause confusion.

Other formats listed, such as HH12, refer to a 12-hour clock system which uses AM and PM to differentiate between morning and evening hours. MM:SS indicates minutes and seconds, but does not include hour representation, and HH:MI is ambiguous but generally also implies a 12-hour format. Consequently, HH24 is the unambiguous choice for representing a full 24-hour time format.