ClockHubClockHubtwenty-one twenty
Military time writes every hour of the day as a two-digit number from 00 to 23, then appends the minutes as two more digits. 2120 therefore means 21 hours and 20 minutes past midnight — 9:20 PM on a 12-hour clock.
Read aloud it becomes "twenty-one twenty". A zero minutes value is spoken as "hundred", which is why whole hours end in "hundred"; minutes below ten take an "oh" in front, as in "seventeen oh five" for 1705.
Twelve-hour clocks split the day at noon. Hours 00 through 11 are AM, hours 12 through 23 are PM, and the two boundary hours are the ones people get wrong: 0000 is 12:00 AM and 1200 is 12:00 PM. 2120 sits on the PM side of that line, which is why it converts to 9:20 PM.
Going the other way is the same arithmetic in reverse. Take 9 and PM, drop the suffix, and add 12 to any PM hour — then watch the two twelve-o’clock cases, where 12 AM becomes 00 and 12 PM stays 12. 9:20 PM becomes 2120.