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The history of the 24 hour day dates back to ancient Babylon and Egypt. The Egyptians and Babylonians divided the sky in 12 sections, and noted 12 groups of stars that would appear at regular intervals. The daytime was divided into 12 hours accordingly.

The dial of a sundial shows the same division of the day in 24 hours, with noon and midnight opposite each other. However, for obvious reasons not many sundials show all 24 markers. Early clocks were modeled after sundials and thus showed 24 hour markers. Two systems were popular; a 24 hour system (1 to 24), and the so called double-XII system (1 to 12, then 1 to 12 again).

Although many countries nowadays use a 24 hour time system, the 24 hour clock dials are very rare. Sometimes both day and night hours are marked on a 12 hour dial. Converting the times from a 24 hour clock to a 12 hour clock is straightforward; for any hour past 12:00 (12:00 PM), you subtract 12 hours to get the corresponding PM time.


12 hour clock 24 hour clock
12:00 AM (midnight) 0:00 1:00 AM 1:00 2:00 AM 2:00 3:00 AM 3:00 4:00 AM 4:00 5:00 AM 5:00 6:00 AM 6:00 7:00 AM 7:00 8:00 AM 8:00 9:00 AM 9:00 10:00 AM 10:00 11:00 AM 11:00 12:00 PM (noon) 12:00 1:00 PM 13:00 2:00 PM 14:00 3:00 PM 15:00 4:00 PM 16:00 5:00 PM 17:00 6:00 PM 18:00 7:00 PM 19:00 8:00 PM 20:00 9:00 PM 21:00 10:00 PM 22:00 11:00 PM 23:00

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