Military time
How do you calculate military time?
Two rules do all of it. In the morning, the hour keeps its number: 9:00 AM is 0900 — “oh-nine-hundred.” From 1:00 PM on, add 12 to the hour: 1:00 PM is 1300, 6:15 PM is 1815, 9:30 PM is 2130. Coming back the other way, subtract 12 from any hour of 13 or more. The minutes never move. Military time is the 24-hour clock with the colon dropped and a zero up front — the same clock with the afternoon un-hidden — and the chart below has every hour already worked out.
| 24-hour | 12-hour | Said as |
|---|---|---|
| 00:00 | 12:00 AM | zero zero hundred hours |
| 01:00 | 1:00 AM | zero one hundred hours |
| 02:00 | 2:00 AM | zero two hundred hours |
| 03:00 | 3:00 AM | zero three hundred hours |
| 04:00 | 4:00 AM | zero four hundred hours |
| 05:00 | 5:00 AM | zero five hundred hours |
| 06:00 | 6:00 AM | zero six hundred hours |
| 07:00 | 7:00 AM | zero seven hundred hours |
| 08:00 | 8:00 AM | zero eight hundred hours |
| 09:00 | 9:00 AM | zero nine hundred hours |
| 10:00 | 10:00 AM | ten hundred hours |
| 11:00 | 11:00 AM | eleven hundred hours |
| 24-hour | 12-hour | Said as |
|---|---|---|
| 12:00 | 12:00 PM | twelve hundred hours |
| 13:00 | 1:00 PM | thirteen hundred hours |
| 14:00 | 2:00 PM | fourteen hundred hours |
| 15:00 | 3:00 PM | fifteen hundred hours |
| 16:00 | 4:00 PM | sixteen hundred hours |
| 17:00 | 5:00 PM | seventeen hundred hours |
| 18:00 | 6:00 PM | eighteen hundred hours |
| 19:00 | 7:00 PM | nineteen hundred hours |
| 20:00 | 8:00 PM | twenty hundred hours |
| 21:00 | 9:00 PM | twenty-one hundred hours |
| 22:00 | 10:00 PM | twenty-two hundred hours |
| 23:00 | 11:00 PM | twenty-three hundred hours |
The two rules, worked through
AM hours: write the hour with two digits and put the minutes straight after. 7:05 AM is 0705; 11:59 AM is 1159. The only odd one is the first hour of the day: 12:30 AM is 0030, because the day starts at zero, not at twelve.
PM hours: add 12. So 1:00 PM is 1300, 4:45 PM is 1645, 11:00 PM is 2300. Noon stays 1200 — it is the last hour that keeps its number. Check yourself against the chart above, or type a time into the converter and see whether you agree.
How to say it
Whole hours are read as hundreds: 0900 is “oh-nine-hundred,” 1300 is “thirteen-hundred.” With minutes, read the pairs: 2130 is “twenty-one thirty.” Nobody says “o’clock” — the number already says which half of the day it is, which is the point.
Why midnight is 0000 and not 2400
A day runs from 0000 to 2359. The moment a clock would read 2400, a new day has begun and it reads 0000 instead. You will occasionally see 2400 on a schedule to mean “the very end of this day” — the same instant as the next day’s 0000, just different bookkeeping.
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Who cannot afford the 12-hour clock
Write 7:00 on a whiteboard and it could be breakfast or dinner — the 12-hour clock throws that information away and trusts context to rescue you. Pilots, nurses, train crews, soldiers and scientists work where context does not rescue you, so they use the clock that cannot be misread. Aviation and the military go a step further and pin the clock to one shared zone: 0900Z — “Zulu” — means 0900 UTC, the same instant everywhere on Earth.