The 24-hour clock is a convention of time keeping in which the day runs from midnight to midnight and is divided into 24 hours, numbered from 0 to 23. This system is the most commonly used time notation in the world today. In the 24-hour time notation, the day begins at midnight, 00:00, and the last minute of the day begins at 23:59.

The 12-hour clock is dominant instead in a handful of countries, particularly the United States and Canada (except Quebec). 24-hour notation is in the US and Canada also popularly referred to as military time or astronomical time, and in Australia (though rarely) as army time. In some parts of the world, it is called railway time. It is also the international standard notation of time (ISO 8601).

U.S. military uses the 24-hour clock exclusively and would typically pronounce full hours as the number-word for the hour followed by "hundred" with an optional "hours" to clarify that the speaker is referring to a time of day. For instance, 16:00 would be pronounced "sixteen hundred" or "sixteen hundred hours". The time 18:30 is usually pronounced "eighteen thirty".

The term military time is a synonym for the 24-hour clock notation, which counts the hours of the day from 00 to 23. In CalcTime, the Military Time is a mirror of the Regular Time Tab and the Military Time Tab serve two purposes. One is to add to times together and the second is to subtract 'B - End' time from 'A - Start' time.