The 24-hour clock system where midnight is 2400, one minute after midnight is 0001 and progresses to 2400 daily. 

Time measured on a twenty-four-hour clock. [D01029]

From 12:00 midnight until 1:00 pm it’s the same, except we say, for example, ten hundred, instead of ten o’clock. One o’clock is 1300, (thirteen hundred), 2 is 1400, 3 is 1500, 4 is 1600, 5 is 1700, 6 [..]

Time that is measured in 24-hour segments. (IE: 10pm is referred to as 2200 hours).

Time measured in 24-hour segments. To convert 12-hour time into 24-hour time, simply add 12 to any p.m. time. To convert 24-hour time into 12-hour time, subtract 12 from any time from 13 to 24.

The easiest way to remember military or Coast Guard time is for any time prior to 10:00 a.m. simply add a zero before the hour, example: nine o’clock in the morning would be spoken as “zero nine hundred” and written as 0900. For any time after 12:00 noon simply add twelve to the time.