(start of day)
(end of day)
shown as start
of next day
|* See section "Confusion
at noon and midnight"
The 12-hour clock is a time convention in which the 24 hours of the day are divided into two periods: a.m. (from the Latin ante meridiem, meaning "before midday") and p.m. (post meridiem, "after midday"). Each period consists of 12 hours numbered: 12 (acting as zero), 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, and 11.
History and use 
The natural day/night division of a calendar day forms the fundamental basis as to why each day is split into two cycles. Originally these were one cycle which could be tracked by the position of the Sun (day) followed by one cycle which could be tracked by the Moon and stars (night). This would eventually evolve into the two 12-hour periods that started at midnight (a.m.) and noon (p.m.) which are used today.
The 12-hour clock can be traced back as far as Mesopotamia and Ancient Egypt: Both an Egyptian sundial for daytime use and an Egyptian water clock for nighttime use were found in the tomb of Pharaoh Amenhotep I. 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".
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. The 12-hour clock was used throughout the British empire.
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 hour hand rotates once every 12 hours and twice in one day. Some analog clock dials have an inner ring of numbers along with the standard 1-to-12 numbered ring. 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 be read also in the 24-hour notation. The 12-hour clock can be found in countries where the 24-hour clock is preferred.
Use by country 
|This section does not cite any references or sources. (January 2013)|
The 12-hour clock is the dominant system of time written and spoken in:
- English Canada
- Costa Rica
- El Salvador
- Hong Kong
- New Zealand
- Saudi Arabia
- United States
The 12-hour clock is mainly used in time spoken, while the 24-hour notation is written, in:
- Czech Republic
- French Canada
- United Kingdom
In many 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. 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 relatively recently; e.g., Rider's British Merlin almanac for 1795 (published in London) uses them, and so does a similar almanac for 1773.
Computer support 
In most countries, computers by default show the time in 24-hour notation. Microsoft Windows and Mac OS activate the 12-hour notation by default only if a computer's language and region settings are:
- Chinese (Singapore and Taiwan)
- English (Australia, Belize, Canada, Caribbean, Jamaica, New Zealand, Philippines, Trinidad, South Africa, United States, and Zimbabwe)
- Japanese (Tokyo and Aomori)
- Korean (Seoul and Pyongyang)
- Spanish (Mexico and parts of South America)
(This can be changed by the user, for example in Windows "Region and Language" settings.)
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, Portuguese and Spanish. 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. However, in many languages, such as Russian and Hebrew, non-formal designations are used, such as "9 in the morning" or "3 in the night".
When abbreviations and phrases are omitted, one may rely on sentence context and societal norms to reduce ambiguity. For example, if one schedules an appointment with a doctor at "9:00", one may mean 9:00 am, but if a social dance is scheduled to begin at "9:00", it may begin at 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. Style guides recommend not using a.m. and p.m. without a time preceding it, although doing so can be advantageous when describing an event that always happens before or after noon.
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", formerly written as 5.5 in some publications in the UK) 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". 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. This form is also used in television and radio broadcasts that cover multiple time zones which are at one hour intervals.
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 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
end of day
|Written 24-hour time,
including ISO 8601
|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
(most common forms)
|U.S. Government Printing Office||12 a.m.||12 p.m.||—|
|U.S. Government Printing Office (1953)||—||12:00 a.m.||12:00 p.m.|
|Japanese legal convention||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||12:01 a.m.||noon||—|
|U.S. de facto legal||12:01 a.m.||—||11:59 p.m.|
|* Digital clocks usually do not 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.
It is not always clear what times "12:00 a.m." and "12:00 p.m." denote. From the Latin words meridies (midday), ante (before) and post (after), the term ante meridiem (a.m.) means before midday and post meridiem (p.m.) means after midday. Since strictly speaking "noon" (midday) is neither before nor after itself, the terms a.m. and p.m. do not apply. However, since 12:01 p.m. is after noon, it is common to extend this usage for 12:00 p.m. to denote noon. That leaves 12:00 a.m. to be used for midnight at the beginning of the day, continuing to 12.01 a.m. that same day.
The American Heritage Dictionary of the English Language has a usage note on this topic: "By convention, 12 AM denotes midnight and 12 PM denotes noon. Because of the potential for confusion, it is advisable to use 12 noon and 12 midnight."
Many U.S. style guides, and NIST's "Frequently asked questions (FAQ)" web page, 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."). Some other style guides suggest "12:00 n" for noon and "12:00 m" for midnight.
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 a 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 these 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.
See also 
- 24-hour clock
- Clock position
- Date and time representation by country
- Decimal time
- Thai six-hour clock
- "Time". The New Encyclopædia Britannica 28. 2005. pp. 670 2a.
- "National Institute of Standards and Technology's Physics Laboratory, Time and Frequency Division FAQ". Retrieved 28 November 2008.
- Susan Addington. "Modular Arithmetic". Archived from the original on 4 July 2008. Retrieved 28 November 2008.
- The History of Clocks
- Berlin instruments of the old Egyptian time of day destination
- A Walk through Time - Water Clocks
- National Library of Australia catalogue entry for Rider's British merlin: for the year of Our Lord God 1795
- Diccionario panhispánico de dudas, HORA (Spanish)
- Hacker, Diana, A Writer's Reference, six edition, Bedford, St Martin's, Boston, 2007, section M4-c, p.308.
- TVTimes. 21–27 May 1983. Retrieved 22 April 2012.
- American Heritage Dictionary, 3rd ed. (Boston: Houghton Mifflin, 1992). s.v. usage note at end of "quarter" entry.
- U.S. Government Printing Office Style Manual: Chapter 12 - Numerals
- 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
- AM at the American Heritage Dictionary of the English Language, Fifth Edition (2011)
- "Times of day, Frequently asked questions (FAQ)". National Institute of Standards and Technology. 18 January 2011. Retrieved 3 May 2012.
- WisDOT. "Wisconsin Occupational Operator License Application". Archived from the original on 2009-04-29. Retrieved 2009-03-07.