What is 0900 in military time?
0900 is 9:00 AM on the 12-hour clock, and 09:00 on the civilian 24-hour clock. It is spoken aloud as “zero nine hundred hours” — never “9 o'clock”, and never with a colon in the written form.
The hour is under 12, so nothing is subtracted. 9 stays 9, and the time is AM.
Written down it is four digits with no separator and no AM or PM: 0900. The first two digits are the hour, the last two are the minutes, and the minutes are ordinary minutes — 0945 would be 9:45 AM.
Where 0900 sits in the day
0900 is 9 hours into the day and 15 hours from the next midnight. In the watch system it falls in the forenoon watch, the main working morning. Briefings, appointments and flight departures are scheduled through this block, and the numbers match the 12-hour clock exactly, so it is the easiest stretch of the day to read.
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 0900 row. For a value that is not on a half hour, the military time converter takes anything you type in either direction, including 9:00 AM and 0900.
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.