Definition
24-hour time
24-hour time gives each hour of the day a number from 00 to 23, so a.m. and p.m. are not needed.
What it means
The hour number keeps counting after noon instead of returning to 1. So 1:00 p.m. becomes 13:00, 2:00 p.m. becomes 14:00, and 11:00 p.m. becomes 23:00. Because each hour number identifies its place in the day, you do not add a.m. or p.m.
Remember it
After noon, keep counting: 13, 14, 15 … up to 23.
Example
A train leaves at 18:40. Since 18 is greater than 12, subtract 12 from the hour: 18 − 12 = 6. The 12-hour time is 6:40 p.m.
Watch for
Military time is related but not identical: it often omits the colon and uses a special way of saying the digits.
Common mix-up
Do not write 18:40 p.m.; the hour 18 already shows that the time is after noon.
Exact meaning
A 24-hour clock numbers the hours of a day from 00 through 23 and the minutes within each hour from 00 through 59. The day begins at 00:00, noon is 12:00, and 23:59 is the final minute before the next day begins.