What is 0100 in military time?
0100 is 1:00 AM on the 12-hour clock, and 01:00 on the civilian 24-hour clock. It is spoken aloud as “zero one hundred hours” — never “1 o'clock”, and never with a colon in the written form.
The hour is under 12, so nothing is subtracted. 1 stays 1, and the time is AM.
Written down it is four digits with no separator and no AM or PM: 0100. The first two digits are the hour, the last two are the minutes, and the minutes are ordinary minutes — 0145 would be 1:45 AM.
Where 0100 sits in the day
0100 is 1 hour into the day and 23 hours from the next midnight. In the watch system it falls in the middle watch, the small hours, when a base is quiet and the only people reading a clock are on duty. Times in this block show up on incident reports, hospital charts and log entries far more often than on anyone's calendar.
The hours either side
Three hours in each direction, since the value you are checking is usually next to the one you actually need.
Everything else on the 24-hour clock
The full chart lists every hour and half hour from 0000 to 2330 and prints on one page — that link jumps straight to the 0100 row. For a value that is not on a half hour, the military time converter takes anything you type in either direction, including 1:00 AM and 0100.
If what actually tripped you up was midnight versus noon rather than this hour, the 12 AM vs 12 PM explainer covers why the 12-hour clock cannot settle that question on its own.