What do AM and PM mean?

12:00 PM is noon. 12:00 AM is midnight, at the start of the day.

Strictly, neither label fits. Noon is not before noon or after noon. Midnight is 12 hours from noon in both directions.

Clocks still have to show one or the other. The US National Institute of Standards and Technology says the first moment after noon is PM. So a clock turns from 11:59 AM to 12:00 PM at noon, and the clock on this site does the same.

That makes 12 PM a convention, not something you can work out from the Latin. The safest choice is to avoid both. Write noon or midnight, or use the 24-hour clock.

Midnight has a second problem. “Midnight on Friday” can mean the start of Friday or the end of it. For deadlines, NIST suggests 11:59 PM or 12:01 AM instead.

The 24-hour clock numbers the hours from 00 to 23, so it needs no AM or PM. The day starts at 00:00, and noon is 12:00.

To convert a PM time, add 12 to the hour. 1:00 PM is 13:00, and 6:00 PM is 18:00. To go the other way, take 12 from any hour above 12, so 23:00 is 11:00 PM.

The end of a day is sometimes written as 24:00. It is the same moment as 00:00 on the next day.

In the United States, the 24-hour clock is often called military time. It is written without the colon, so 1:00 PM is 1300 and midnight is 0000.

The clock on the home page switches between the 12-hour and 24-hour formats with its 12 / 24H button. Your browser remembers the choice.

Every hour of the day on both clocks, with the military form.