The simplest way to convert civilian a.m./p.m. time to military time is to add 12 to every hour after 12 p.m. For example, 4 p.m. becomes 1600 hours. All a.m. hours are unchanged but have an "O" prefix and "hundred hour(s)" suffix instead of the typical "o'clock."
In military time, midnight is zero hours. For simplicity in record keeping, military time is also used in the medical field. Computers also use the 24-hour system for communications. Military time is broken into 24 alphabetic time zones. To coordinate forces in different time zones, all operational times are given in Greenwich Mean Time, which is time zone "Zulu."