Meaning of AM and PM

AM stands for “ante meridiem”, which means “before noon” in Latin, while PM stands for “post meridiem”, which means “after noon” in Latin.

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.