12-hour clock

"AM and PM" redirects here. For other uses, see AM PM (disambiguation).
"Ante meridiem" redirects here. For the meridian 180° both east and west of the prime meridian in a geographical coordinate system, see antimeridian.

The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from Latin ante meridiem, translating to "before midday") and p.m. (from Latin post meridiem, translating to "after midday").[1][2] Each period consists of 12 hours numbered: 12 (acting as 0),[3] 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11. The 12-hour clock has been developed since the second millennium BC and reached its modern form in the 16th century.

The 12-hour time convention is common in several English-speaking nations and former British colonies, as well as a few other countries. There is no widely accepted convention for how midday and midnight should be represented: in English-speaking countries, "12 p.m." indicates 12 o'clock noon, while "12 a.m." means 12 o'clock midnight.[4][5][6]

Clock system
12-hour 24-hour
Midnight (start of day)
12 midnight
12:00 a.m.[a]		 00:00
12:01 a.m. 00:01
 1:00 a.m. 01:00
11:00 a.m. 11:00
11:59 a.m. 11:59
Noon
12 noon
12:00 p.m.[a]		 12:00
12:01 p.m. 12:01
 1:00 p.m. 13:00
11:00 p.m. 23:00
11:59 p.m. 23:59
Midnight (end of day)
or shown as start of next day[a]		 24:00

(00:00)

  1. ^ a b c See: §Confusion
    at noon and midnight
  1. ^ "Time". The New Encyclopædia Britannica. Vol. 28. 1986. pp. 660 2a.
    "Time". Encyclopædia Britannica. Encyclopædia Britannica Online Library Edition. Retrieved 20 November 2013. The use of AM or PM to designate either noon or midnight can cause ambiguity. (subscription required) For different opinions on representation of midday and midnight, see #Confusion at noon and midnight
  2. ^ Cite error: The named reference NISTFAQ was invoked but never defined (see the help page).
  3. ^ Addington, Susan (25 August 2016). "Modular Arithmetic". Archived from the original on 4 July 2008. Retrieved 28 November 2008.
  4. ^ "Is noon 12 am or 12 pm?". Royal Museums Greenwich. Retrieved 26 January 2023.
  5. ^ "U.S. Government Printing Office Style Manual - Chapter 12 - Numerals". govinfo.gov. Retrieved 27 January 2023.
  6. ^ "time of day, elapsed time". Resources of the Language Portal of Canada. 22 October 2020. Retrieved 27 January 2023.