0100 Military Time

0100 is 1:00 AM, one o’clock in the night. On a 24-hour clock with a colon it is 01:00; in military style the colon goes and the four digits stand alone. The hour is 1 hour after midnight and 23 hours before the next one.

The first full hour after midnight. The leading zero is not decoration: 0100 is one in the morning, while a bare 100 is not a time at all.

To read it, look at the first two digits: the hour is below 12, so drop the leading zero and add AM: 1 o’clock in the morning. The last two digits are minutes and never change, so 0125 is 1:25 AM and 0150 is 1:50 AM.

Military to 12-hour. Split the four digits into hours and minutes, the hour is below 12, so drop the leading zero and add AM: 1 o’clock in the morning. So 0115 becomes 1:15 AM and 0145 becomes 1:45 AM.

12-hour to military. 1 AM keeps its number and gains a leading zero so that it has four digits: 1:05 AM is 0105. Drop the colon and the AM or PM, and you have the four digits.

The mistakes worth knowing: “01:00 AM” is a mix of both systems and means nothing, because a 24-hour time already says which half of the day it is. And 0100 is not 1 PM; 1 PM is 1300.

0100 is said “zero one hundred”. The “hundred” stands in for the two zeros; nobody says “thirteen zero zero”. The leading zero is spoken: “zero one hundred”. “Oh one hundred” is common in conversation and in films, but “zero” is what radio procedure and most instructors use, because “oh” can be misheard.

Minutes follow the same logic. 0105 is “zero one zero five” and 0140 is “zero one forty”. Single-digit minutes get a spoken zero, so the listener knows a digit is there.

About the word “hours”: in speech, “zero 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 0100”, not “at 0100 hours”.

On its own, 0100 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 0100Z 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.