Military time, explained in one rule

The 24-hour clock scares people out of proportion to its content, which is one subtraction. Morning times are identical in both systems; afternoon and evening times are the familiar number plus twelve. That's the whole system — the rest of this page is practice, edge cases and the reasons hospitals, militaries and airlines refuse to use anything else.

The one rule

Reading: 13:00 or higher? Subtract 12 and it's PM. 1300 − 12 = 1 PM; 1830 = 6:30 PM; 2245 = 10:45 PM. Below 1200 it reads as normal morning time, with a leading zero: 0700 is 7 AM.

Writing: the reverse — any PM time except 12 PM, add 12. 3:15 PM → 1515. Noon is 1200 as-is, and 12-anything AM (the after-midnight hour) becomes 00-anything: 12:30 AM → 0030.

Those two twelve o'clocks are the only places anyone stumbles, because "12 AM" and "12 PM" are themselves the confusing ones — the 24-hour clock is the system where they're unambiguous.

The full conversion chart

The memorable landmarks that anchor the whole afternoon: 1500 is 3 PM, 1800 is 6 PM, 2100 is 9 PM. Know those three and everything else is within an hour of an anchor. (Our digital clock switches between 12- and 24-hour display, which is the fastest way to build the intuition — leave it on 24 for a week.)

Midnight: 0000 or 2400?

Both exist, with different jobs. 0000 starts a day; 2400 ends one. A shift written 1800–2400 ends at the stroke of midnight Tuesday; one starting 0000 begins Wednesday. Most timekeeping (and ISO 8601) standardises on 0000, and 2400 survives mainly in schedules and contracts, precisely for that end-of-this-day meaning. One minute after either is 0001.

How to say it aloud

Military convention reads the digits with "hundred" for whole hours: 0700 is "oh-seven-hundred" (or strictly "zero-seven-hundred"), 1300 is "thirteen-hundred", and minutes read straight through — 1845 is "eighteen forty-five". There is no "hours o'clock" and, outside of movies, "hundred hours" attached to minutes (as in "eighteen forty-five hours") is said far more often by Hollywood than by anyone on a radio.

Why anyone uses it

Because AM/PM is an error factory in any context where a twelve-hour mistake costs something. A medication order at "8:00" is dangerous in a way "0800" cannot be; flights, trains, militaries and every hospital chart use 24-hour time for the same reason. It's also simply how most of the world writes timetables — the US, with pockets elsewhere, is the exception, not the rule. If you work across borders, the 24-hour clock is the shared dialect, and zone-aware scheduling usually gets written in it.

Zulu time

Aviation and the military add one more layer: everything is coordinated in UTC, call-signed Zulu (Z for the zero meridian, spoken in the NATO alphabet). "1500Z" is 1500 UTC everywhere on Earth at the same moment — a pilot crossing five time zones files, flies and logs in a single one. Converting Zulu to your wall clock is a time-zone problem, which is exactly what the time zone converter is for.