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12-Hour Periods

Most clocks today are 12-hour clocks – they divide the 24 hours in a day into two 12-hour periods.

§  Ante meridiem: Before noon
Between midnight (0:00) & noon (12:00)

§  Post meridiem: After noon
Between noon (12:00) & midnight (0:00)

Ante meridiem is commonly denoted as AM, am, a.m. or A.M. while post meridiem is usually abbreviated to PM, pm, p.m. or P.M.

Each hour during the day, except midnight and noon, is suffixed by am or pm to identify whether the hour being referred to is in the morning, afternoon or evening.

Hours Passed Since Midnight A 24-hour clock displays the time as the hours passed since midnight. So, 23:00 hours on a 24-hour clock on February 15, means 23 hours have passed since midnight of February 15.

The 24-hour time format is also sometimes known as military time. Ancient Origins The Egyptians are thought to be responsible for dividing the day into 24 equal parts. One account suggests that this is because the Egyptians used the base 12, since they counted the 3 joints in their fingers, not including the thumb. Other accounts point towards the use of a system of 36 groups of stars, called decans by the Egyptians to tell time at night. The star groups in the decans tend to rise in the sky 40 minutes after one another.

12 pm or Noon? Technically, 12 pm does not make sense, since it is not after noon. Similarly, 12 am does not make much sense either, since it is not before noon. It is recommended to refer to these hours as midnight and noon. Note: Most digital clocks tend to show 12 am and 12 pm to specify midnight and noon, respectively.

Midnight Confusion If you were asked to pick up a friend at the airport at midnight of April 13, when would you go to pick her up – midnight at the beginning of April 13, or midnight at the end of April 13? Time Formats 12-hour 24-hour 12:00 (Midnight) 0:00 (beginning of the day) or 24:00 (end of day) 12:01 am 0:01 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 (Noon) 12:00 12:01 pm 12:01 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 Referring to midnight can sometimes be confusing, because it could be taken to mean as midnight at the beginning of a day or midnight at the end of the day. Technically, it is the time when the date changes from the previous date to the next date. One way to overcome this problem is to refer to the time as 12:01 am, April 13 or 11:59 pm April 13 depending on the time you are referring to. Or use the 24-hour clock format to refer to the time. For instance, if your friend asked you to pick her up at 00:00 on April 13, you shuld be at the airport at the beginning of April 13, while if she told you to pick her up at 24:00 on April 13, you must be at the airport at the end of April 13. AM or PM to 24-Hour Clock You can convert am or pm time to time on a 24-hour Clock by using these rules. From midnight to 12:59 am, subtract 12 hours. 12:49 am is 12:49 – 12 = 0:49 From 1 am to noon, do nothing. 11:49 am is 11:49 From 12:01 pm to 12:59 pm, do nothing. 12:49 pm is 12:49 From 1:00 pm to midnight, add 12 hours. 1:49 pm is 1:49 +12 = 13:49 24-hour Clock to AM or PM Here’s how you can convert time on a 24-hour Clock to am or pm. From 0:00 (midnight) to 0:59, add 12 hours, and am after the time. 0:49 is 0:49 + 12 = 12:49 am From 1:00 to 11:59, just add am after the time. 11:49 is 11:49 am From 12:00 to 12:59, just add pm after the time. 12:49 is 12:49 pm From 13:00 to 0:00, subtract 12 hours, and add pm after the time. 13:49 is 13:49 -12 = 1:49 pm Topics: Timekeeping, Clocks

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