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.

The 12-hour clock is a timekeeping convention in which the 24 hours of the day are divided into two periods called ante meridiem (a.m., Latin "before noon") and post meridiem (p.m., "after noon"). Each period consists of 12 hours numbered 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. Noon and midnight are used to indicate 12:00 in the 12-hour system.

The 12-hour clock was developed over time from the mid-second millennium BC, to the 16th century A.D. and was once popular throughout Northern Europe, but is now used as the dominant system in only a few former British and American Colonies. It is also used informally in most of the world. The notion has received much criticism, yet survives out of tradition.