One number, no AM or PM
Military time is the 24-hour clock written without a colon: the hours run 00 through 23, so every instant in the day has exactly one four-digit representation and the AM/PM ambiguity disappears. This is the whole point. A written order for “7:00” is genuinely ambiguous; 0700 is not.
Morning is the easy half
From 1 AM to 11:59 AM the numbers match: 0900 is 9:00 AM. From 1 PM onward you add twelve, so 1:45 PM becomes 1345. The two awkward cases sit at the boundaries.
Midnight and noon
Midnight is 0000 and noon is 1200 — the reverse of what the 12-hour clock suggests, where midnight is confusingly 12:00 AM and noon is 12:00 PM. The 12-hour convention here is genuinely poor design, since AM means ante meridiem, before noon, and 12:00 PM is the meridiem itself rather than after it.
Midnight has a second wrinkle: it can be written 0000 or 2400 depending on whether you mean the start of a day or the end of the previous one. They are the same instant. Contracts and duty rosters use 2400 to close a period and 0000 to open one, which avoids arguments about which day a deadline fell on.
How it is spoken
Military time is read digit-group by digit-group: 1345 is “thirteen forty-five”, and on the hour 1300 is “thirteen hundred”. Minutes under ten take an “oh”: 1305 is “thirteen oh five”. Note that most of the world uses the 24-hour clock in civilian life and simply writes it as 13:45; the colon-free four-digit form and the word “hours” are the specifically military conventions.