In the days before inexpensive clocks, wrist watches, and PA systems, the time had to be kept track of and announced using the ship’s bell. Historically a ship’s boy would be stood next to a half-hour glass, turning it every time it emptied. Then he would ring the bell to announce the time to the crew. Each half hour another toll would be added up until four hours had elapsed – this was the length of a watch, and 8 bells was the signal to either begin the next watch of work or to take the next watch off (after being properly relieved!). After 8 bells, the system restarts at 1 bell come the next half hour.
Today we keep track of time onboard using any number of available clocks, wrist watches, and so forth. However for clarity of record keeping and communication, because the vessel operates around the clock we use the 24-hour clock system of keeping time, commonly referred to as military time.
First, a quick review. Civilian time is broken down into AM (“ante-meridian” basically meaning “before noon”) and PM (“post-meridian” or “after noon”). After noon we say 1pm, 2pm, and so forth. This is where military time makes its departure.
In military time, 1 o’clock in the morning is 0100 hours, 2 o’clock is 0200 hours, and so on until noon. At this point it just keeps counting. To convert quickly in your head, add 12 hours to any time after noon on the civilian clock. 1 o’clock in the afternoon becomes 1300 hours, 2 o’clock becomes 1400 hours, all the way until midnight. This is useful to know if you read time on a 12-hour wrist watch.
An import point to make: there is no 2400 hours. When a minute is added to the clock at 23:59 it switches over to 00:00 the following day. This is colloquially referred to as “balls up” because, well, it looks like four balls.