2100 Military Time

2100 is 9:00 PM, nine o’clock in the night. On a 24-hour clock with a colon it is 21:00; in military style the colon goes and the four digits stand alone. The hour is 21 hours after midnight and 3 hours before the next one.

Nine in the evening. 2100 is lights-out for trainees and curfew in many barracks.

To read it, look at the first two digits: the hour is above 12, so subtract 12 and add PM: 21 − 12 = 9. The last two digits are minutes and never change, so 2125 is 9:25 PM and 2150 is 9:50 PM.

Military to 12-hour. Split the four digits into hours and minutes, the hour is above 12, so subtract 12 and add PM: 21 − 12 = 9. So 2115 becomes 9:15 PM and 2145 becomes 9:45 PM.

12-hour to military. 9 PM is a PM hour, so add 12: 9 + 12 = 21. 9:05 PM is 2105. Drop the colon and the AM or PM, and you have the four digits.

The mistakes worth knowing: “21:00 PM” is a mix of both systems and means nothing, because a 24-hour time already says which half of the day it is. And 2100 is not 21:00 AM; there is no 21 o’clock in the morning.

2100 is said “twenty-one hundred”. The “hundred” stands in for the two zeros; nobody says “thirteen zero zero”. Numbers from ten upward are said as ordinary numbers: “twenty-one hundred”, not “one one hundred”.

Minutes follow the same logic. 2105 is “twenty-one zero five” and 2140 is “twenty-one forty”. Single-digit minutes get a spoken zero, so the listener knows a digit is there.

About the word “hours”: in speech, “twenty-one hundred hours” is everywhere, and nobody will correct you. In writing, Army Regulation 25-50 says the word is not used with military time, so a memorandum reads “at 2100”, not “at 2100 hours”.

On its own, 2100 is local time wherever it is written. When the four digits carry a letter, the letter is the time zone. Z stands for Zulu, which is UTC, and J (Juliet) means the local time of the person speaking. The table converts 2100Z to the four US zones, in standard time and in daylight saving time.

Eastern Time is four hours behind UTC from March to November and five hours behind the rest of the year, which is why one Zulu time maps to two possible local times. The other three zones follow the same pattern an hour further west each.

Midnight has a second spelling, 2400, used when it marks the end of a day rather than the start. The full rules, with a converter, are on the military time chart page.