On a 12-hour clock the hours run from 1 to 12 twice a day, so we tag each one with am (morning) or pm (afternoon and evening). The 24-hour clock does away with that: it counts the hours straight through from 00 (midnight) to 23, so there is no am or pm at all.

Times are written as four digits — two for the hour, two for the minutes — like 09{:}00, 14{:}30 and 21{:}15.

Morning times (am) keep the same hour number: 8\text{ am} = 08{:}00. Afternoon and evening times (pm) add 12 to the hour: 3\text{ pm} = 15{:}00 and 9\text{ pm} = 21{:}00. Two times stand on their own: noon is 12{:}00 and midnight is 00{:}00.