What does 21 military time mean in civilian time?

In military time, 21 means 2100 or 9 PM. Military time operates on a 24-hour clock, so times beginning after 12 AM are like the following: 1 AM to 9 AM is 0100 to 0900, 10 AM to 11 PM is 1000 to 2300, and 12 PM to 11 PM is 1200 to 2300.

The 24-hour clock begins at midnight or 0000 hrs. From 1 AM to 9 PM, a zero appears before the numeral signifier. 10 AM to noon begin with 10, 11, 12. In the afternoon and evening, the 24-hour clock adds traditional time to 1200, so 1 PM becomes 1300 hrs and 9 PM becomes 2100 hrs. Despite being called a 24-hour clock, the clock ends at 2359, or 11:59 PM.