Convert between the 24-hour clock and the 12-hour AM/PM clock — including the midnight and noon edge cases that catch almost everyone.
Military time is the 24-hour clock written without a colon: 0930 is half past nine in the morning, 1730 is half past five in the afternoon. For any hour after noon you subtract twelve to get the 12-hour equivalent, and for any hour before noon the two notations agree.
The parts that catch people are the boundaries. Midnight is 0000 and noon is 1200, which means 12 AM and 12 PM are the two labels most likely to be written the wrong way round.
Subtract twelve from any hour past noon.
A quarter past midnight — the hour is 00, but the 12-hour label is 12 AM.
Quarter to one in the afternoon. Noon is 12 PM, not 12 AM.
Crossing midnight rolls to the next day rather than to 24:01.
5:30 PM. For any time from 1300 onward, subtract 12 from the hour: 1730 - 1200 = 530, so 5:30 in the afternoon.
Both notations exist, but 0000 is the standard start of a day and is what you should write. 2400 is occasionally used to mean the end of a day, which makes it ambiguous — 0000 on the following date is unambiguous.
Noon. 12 PM is midday and 12 AM is midnight, which is the reverse of what most people guess. This is exactly why the 24-hour clock is preferred in aviation, healthcare and the military: 1200 and 0000 cannot be confused.
Conventionally yes — 0930 is spoken as "zero nine thirty". Written form keeps the leading zero too, so nine in the morning is 0900 rather than 900.