12-hour clock

12-hour clock
Clock system
12-hour 24-hour
(start of day)
12:59 a.m. 00:59
  1:00 a.m. 01:00
  2:00 a.m. 02:00
10:00 a.m. 10:00
11:00 a.m. 11:00
11:59 a.m. 11:59
12:00 noon* 12:00
12:01 p.m. 12:01
12:59 p.m. 12:59
  1:00 p.m. 13:00
  2:00 p.m. 14:00
10:00 p.m. 22:00
11:00 p.m. 23:00
11:59 p.m. 23:59
(end of day)
shown as start
of next day
* See section "Confusion
at noon and midnight

The 12-hour clock is a time conversion convention in which the 24 hours of the day are divided into two periods[1] called ante meridiem (a.m., English: "before midday") and post meridiem (p.m., English: "after midday").[2] Each period consists of 12 hours numbered: 12 (acting as zero),[3] 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11.

The 12-hour clock was developed over time from the mid-second millennium BC to the 16th century AD.


History and use

Exeter Cathedral clock, showing the Double-XII numbering scheme.

The 12-hour clock can be traced back as far as Mesopotamia and Ancient Egypt:[4] Both an Egyptian sundial for daytime use[5] and an Egyptian water clock for night time use were found in the tomb of Pharaoh Amenhotep I.[6] Dating to c. 1500 BC, these clocks divided their respective times of use into 12 hours each.

The Romans also used a 12-hour clock: daylight was divided into 12 equal hours (of, thus, varying length throughout the year) and the night was divided into four watches. The Romans numbered the morning hours originally in reverse. For example, "3 a.m." or "3 hours ante meridiem" meant "three hours before noon", compared to the modern usage of "three hours into the first 12-hour period of the day".[citation needed]

The first mechanical clocks in the 14th century, if they had dials at all, showed all 24 hours, using the 24-hour analog dial, influenced by astronomers' familiarity with the astrolabe and sundial, and their desire to model the apparent motion of the Sun. In Northern Europe these dials generally used the 12-hour numbering scheme in Roman numerals, but showed both a.m. and p.m. periods in sequence. This is known as the Double-XII system, and can be seen on many surviving clock faces, such as those at Wells and Exeter. Elsewhere in Europe, particularly in Italy, numbering was more likely to be based on the 24-hour system (I to XXIV), reflecting the Italian style of counting the hours.

During the 15th and 16th centuries, the 12-hour analog dial and time system, with its simpler and more economical construction, gradually became established as standard throughout Northern Europe for general public use. The 24-hour analog dial was reserved for more specialized applications, such as astronomical clocks and chronometers.

Most analog clocks and watches today use the 12-hour dial, on which the shorter and sometimes thicker hour hand rotates once every 12 hours and twice in one day. Some analog clock dials have an inner ring of numbers paired with the standard 1 to 12 ring of numbers. The number 12 is paired either with a 00 or a 24, while the numbers 1 through 11 are paired with the numbers 13 through 23, respectively. This modification allows the clock to also be read in the 24-hour notation. The 12-hour clock can be found in countries where the 24-hour clock is preferred.

Use by country

Although it has largely been replaced today by the 24-hour notation around the world,[citation needed] especially in written communication, the 12-hour notation with a.m. and p.m. suffixes is common in some parts of the world.

In most other countries the 12-hour clock is used in speech alongside the 24-hour clock.[citation needed]

In many European countries and Western countries, the 12-hour clock is commonly used in informal speech with descriptive phrases such as in the morning, in the afternoon, in the evening, and at night.[citation needed] Outside of English-speaking countries, the terms a.m. and p.m. are seldom used and often unknown. In England, the descriptive phrases were universal until comparatively recently - Rider's British Merlin almanack for 1795 (published in London) uses them, and so does a similar almanack for 1773.

A typical digital 12-hour alarm clock indicating p.m. with a dot to the left of the hour


The Latin abbreviations a.m. and p.m. (often written "am" and "pm", "AM" and "PM", or "A.M." and "P.M.") are used in English and Spanish.[7] The equivalents in Greek are π.µ. and µ.µ., respectively.

Most other languages lack formal abbreviations for "before noon" and "after noon", and their users use the 12-hour clock only orally and informally.[citation needed]

In practice, when abbreviations and phrases are omitted, one relies on sentence context and common sense to distinguish the meaning. For example, if one schedules an appointment with a doctor at "9:00" on a certain date, that means 9:00 a.m.; but if a social dance is scheduled to begin at "9:00", it means 9:00 p.m.

Related conventions


The terms "a.m." and "p.m." are abbreviations of the Latin ante meridiem (before midday) and post meridiem (after midday). Depending on the style guide referenced, the abbreviations "AM" and "PM" are variously written in small capitals ("am" and "pm"), uppercase letters ("AM" and "PM"), or lowercase letters ("am" and "pm").

There are symbols for "a.m." (U+33C2 = "㏂") and "p.m." (U+33D8 = "㏘") in Unicode. They are meant to be used only with Chinese-Japanese-Korean character sets, however, as they take up exactly the same space as one Chinese character.

Some stylebooks suggest the use of a space between the number and the a.m. or p.m. abbreviation.[citation needed] Style guides recommend not using a.m. and p.m. without a time preceding it,[8] although doing so can be advantageous when describing an event that always happens before or after noon.

The hour/minute separator varies between countries: some use a colon, others use a period (full stop).

Informal speech and rounding off

It is common to round a time to the nearest five minutes and express the time as so many minutes past an hour (e.g., 5:05 is "five past five" or "five oh five") or minutes to an hour (e.g., 5:55 is "five to six"). The period 15 minutes is often expressed as "a quarter" (hence 5:15 is "a quarter past five") and 30 minutes is expressed as "half" (hence 5:30 is "half past five" or merely "half five", the latter expression not being common in the USA). The time 8:45 is spoken as "(a) quarter to (or of, before, or til) nine".[9] Moreover, in situations where the relevant hour is obvious or has been recently mentioned, speakers can state simply "quarter to", "half past", etc., to avoid elaborate sentences in particularly informal conversations.

Instead of meaning 5:30, the "half five" convention is sometimes used to mean 4:30, i.e., "half-way to five", especially in the more German-influenced parts of the U.S.A (the Midwest, essentially). "Half-way to five" follows the usage in German speaking countries. It is also found in Indonesian, Norwegian, Danish, Swedish, Dutch, Serbian, Estonian, Latvian, Lithuanian, Polish, Czech, Slovak, Finnish, Hungarian, Russian and Afrikaans.

Formal speech and times to the minute

Minutes may be expressed as an exact number of minutes past the hour specifying the time of day (e.g., 6:32 p.m. is "thirty-two minutes past six in the evening").

Times of day ending in ":00" minutes (full hours) are often said in English as the numbered hour followed by o'clock (10:00 as ten o'clock, 2:00 as two o'clock). This may be followed by the "a.m." or "p.m." designator, though phrases such as in the morning, in the afternoon, in the evening, or at night more commonly follow analog-style terms such as o'clock, half past three, and quarter to four. O'clock itself may be omitted, telling a time as four a.m. or four p.m. Minutes ":01" to ":09" are usually pronounced as oh one to oh nine (nought or zero can also be used instead of oh). Minutes ":10" to ":59" are pronounced as their usual number-words. For instance, 6:02 a.m. can be pronounced six oh two a m; 6:32 a.m. could be told as six thirty-two a.m.

U.S. military speech and writing

See Military time

Confusion at noon and midnight

Time as denoted by various devices or styles
Device or style Midnight
start of day
Noon Midnight
end of day
Written 24-hour time,
including ISO 8601
00:00 12:00 24:00
24-hour digital clocks 00:00 12:00 — *
12-hour digital clocks
with a.m. and p.m.
12:00 a.m. 12:00 p.m. — *
Written 12-hour time ** ** — **
U.S. Government Printing Office[10] 12 a.m. 12 p.m.
U.S. Government Printing Office (1953) 12:00 a.m. 12:00 p.m.
Japanese legal convention[11] 0:00 a.m. 12:00 a.m. 12:00 p.m.
Antiquated † 12:00 m.n. 12:00 m. 12:00 m.n.
Canadian Press, UK standard, NIST1 midnight noon midnight
NIST2 12:00 Midnight 12:00 Noon 12:00 Midnight
Associated Press Style[12] 12:01 a.m. noon
U.S. de facto legal 12:01 a.m. 11:59 p.m.
Encyclopædia Britannica[1] Midnight
December 11–12
12m Midnight
December 12–13
* Digital clocks never reach midnight at the end of the day. Instead they wrap from 11:59 p.m. or 23:59 to midnight at the start of the next day.
Likewise the written 12-hour style wraps immediately to the start of the next day.
† These styles are ambiguous with respect to whether midnight is at the start and or end of each day.

Since the Latin word meridies means noon or midday, it is inconsistent to refer to noon as either "12 a.m." ("12 ante meridiem", or "12 o'clock before noon") or as "12 p.m." ("12 post meridiem", or "12 o'clock after noon"). On the other hand, midnight could logically be called either "12 p.m." (12 post meridiem, 12 hours after the previous noon) or "12 a.m." (12 ante meridiem, 12 hours before the following noon); "x a.m." no longer means "x hours before noon", but the x-numbered hour before noon.

The National Maritime Museum, Greenwich, states:

To avoid confusion, the correct designation for twelve o'clock is 12 noon or 12 midnight. Alternatively, the twenty-four-hour-clock system may be used. The abbreviation a.m. stands for ante-meridiem (before the sun has crossed the line) and p.m. for post-meridiem (after the sun has crossed the line). At 12 noon the sun is at its highest point in the sky and directly over the meridian. It is therefore neither "ante-" nor "post-".[13]

However, as discussed elsewhere in the same reference, the sun is highest at 12 noon local Solar time, not 12 noon civil time, the difference being given by the equation of time[14] plus the effect of time zones and of the daylight saving time.

The American Heritage Dictionary of the English Language (Fourth Edition, 2000) has a similar usage note on this topic: "Strictly speaking, 12 a.m. denotes midnight, and 12 p.m. denotes noon, but there is sufficient confusion over these uses to make it advisable to use 12 noon and 12 midnight where clarity is required."[15]

Many U.S. style guides (including the NIST website) recommend that it is clearest if one refers to "noon" or "12:00 noon" and "midnight" or "12:00 midnight" (rather than to "12:00 p.m." and "12:00 a.m.", respectively). Some other style guides suggest "12:00 n" for noon and "12:00 m" for midnight,[16] but that conflicts with the older tradition of using "12:00 m" for noon[1] (Latin meridies), and "12:00 mn" for midnight (Latin media nox).

The Canadian Press Stylebook (11th Edition, 1999, page 288) says, "write noon or midnight, not 12 noon or 12 midnight." Phrases such as "12 a.m." and "12 p.m." are not mentioned at all.

The use of "12:00 midnight" or "midnight" is still problematic because it does not distinguish between the midnight at the start of a particular day and the midnight at its end. To avoid confusion and error, some U.S. style guides recommend either clarifying "midnight" with other context clues, or not referring to midnight at all. For an example of the latter method, "midnight" is replaced with "11:59 p.m." for the end of a day or "12:01 a.m." for the start of the next day. That has become common in the United States in legal contracts and for airplane, bus, or train schedules, though some schedules use other conventions.

The 24-hour clock notation avoids all of those ambiguities by using 00:00 for midnight at the start of the day and 12:00 for noon. From 23:59:59 the time shifts (one second later) to 00:00:00, the beginning of the next day. In 24-hour notation 24:00 can be used to refer to midnight at the end of a day.

In Britain, various conventions are employed. TV mag, "Sun" newspaper, London, 17 December 2005 uses "noon (12.00)" and "midnight (0.00)" in individual listings. Sequential listings start with a.m. or p.m. as appropriate, but these indicators are not used again, although in sub-listings "12midnight" is sometimes employed. The London Daily Telegraph uses "12.00noon" and "12.00midnight" in individual listings. In sequential listings the first programme to start after 12.00 is marked "am" or "pm" as appropriate. If a station comes on - air at 12.00 the time is marked "12 00 noon". If it goes off - air at 12.00 the time is marked "12.00 midnight".

See also


  1. ^ a b c "Time". The New Encyclopædia Britannica. 28. 2005. pp. 670 2a. 
  2. ^ "National Institute of Standards and Technology's Physics Laboratory, Time and Frequency Division FAQ". http://www.nist.gov/physlab/div847/faq.cfm. Retrieved 28 November 2008. 
  3. ^ Susan Addington. "Modular Arithmetic". Archived from the original on 4 July 2008. http://web.archive.org/web/20080704121745/http://www.math.csusb.edu/faculty/susan/number_bracelets/mod_arith.html. Retrieved 28 November 2008. 
  4. ^ The History of Clocks
  5. ^ Berlin instruments of the old Egyptian time of day destination
  6. ^ A Walk through Time - Water Clocks
  7. ^ Diccionario panhispánico de dudas, HORA (Spanish)
  8. ^ Hacker, Diana, A Writer's Reference, six edition, Bedford, St Martin's, Boston, 2007, section M4-c, p.308.
  9. ^ American Heritage Dictionary, 3rd ed. (Boston: Houghton Mifflin, 1992). s.v. usage note at end of "quarter" entry.
  10. ^ Frwebgate.access.gpo.gov
  11. ^ Japanese Page
  12. ^ Ed. Norm Goldstein, The Associated Press Stylebook and Briefing on Media Law: with Internet Guide and Glossary, P.161, 177, Perseus Publishing, 2002, Cambridge, Massachusetts, United States, LCCN 2002105974, ISBN, 0-7382-0740-3
  13. ^ National Maritime Museum: sea, ships, time and the stars : NMM
  14. ^ The equation of time : Time & timekeeping : Fact files & in-depth : Learning : NMM
  15. ^ A.M.
  16. ^ WisDOT. "Wisconsin Occupational Operator License Application". Archived from the original on 2009-04-29. http://www.dot.wisconsin.gov/drivers/docs/mv3027-sample.pdf. Retrieved 2009-03-07. 

External links

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