|What time is it? 2017, of course.|
It's common for seagoing vessels to use a 24-hour clock since most are 24-hour a day operations. It's easier to say that something is going to happen at 0700 or 1900 rather than risk confusion by saying it's going to happen at 7 o'clock and not be sure if that means am or pm. For those of us raised with a twelve-hour clock, this can be confusing at first and you have to convert the time in your head. To figure out what time 1610 is you subtract twelve hours from 16 to get 4, and you know it's "pm" because 16 is greater than 12 (1200 is noon). It's like learning a foreign language, though: after awhile you stop translating and just start thinking in 24-hour time.
Watches. Time on ship is also measured by watches, work periods meaning the same thing as a "shift" for land occupations. In the early Age of Sail, ship's crews were divided into two groups, also called watches, which switched off every four hours. William Bligh, of HMS Bounty fame, came up with a three-watch system of which a version is often still used today:
- Middle Watch: Midnight to 0400
- Morning Watch: 0400 to 0800
- Forenoon Watch: 0800 to 1200
- Afternoon Watch: 1200 to 1600
- First Dog Watch: 1600 to 1800
- Second Dog Watch: 1800 to 2000
- First Watch: 2000 to Midnight
Each watch was further subdivided by bells. The ship's bell was sounded once a half hour into the watch, twice one hour into the watch, and so on with eight bells marking the end of one watch and the beginning of another (or four bells in the case of the two hour dog watches).
In 1915, the US Congress mandated a change in the watch system for US-flagged vessels, consolidating the dog watches into one four-hour watch and requiring a three-watch system on most vessels. Thus, most crew members will work one four-hour watch, then have two off, thus working two four-hour watches each day (although there is often more work available or required during technically off-watch periods). Just to add to the confusion, these watches usually correspond to the same am and pm times on a 12-hours clock, so a crew member on watch 0400-0800 and 1600-2000 will say he or she stands the "four-to-eights."