2100 military time

2100 is 9:00 PM. On military installations this is often when lights-out begins for trainees in basic training, and it sits inside the window when "Taps" — the bugle call that signals the end of the day — is traditionally sounded (2100 or 2200, depending on the branch and post). It is the hour the day starts formally shutting down.

In healthcare, 2100 is frequently the final major scheduled medication time before the overnight period, making accurate documentation especially important as staffing thins out for the night shift. A dose recorded as "9:00" instead of 2100 could be misread as 0900 the next morning — a twelve-hour error on the last hand-off of the day.

21 − 12 = 9, so 2100 is 9:00 PM, spoken as "twenty-one hundred hours." In countries that use the 24-hour clock by default, 21:00 is simply when prime-time television begins and evening events are scheduled — the same instant, written the clearer way.