The hours between 12 midnight and 12 noon are A.M. (ante “before” meridiem).
The hours between 12 noon and 12 midnight are P.M. (post “after” meridiem)
Sailor time is used on a ship at sea. The day is divided into 5 “watches” of 4 hours each and 2 of 2 hours each. Bells tell the time. One bell marks the first half-hour of the watch; 8 bells toll the last. On ship, the new day starts at noon.
Military time is based on a 24-hour clock, making 3:00 P.M. 1500 hours.