View source for Physics:12-hour clock

{{Short description|Time counting system}}
{{Redirect|AM and PM|other uses|AM PM (disambiguation)}}
{{Redirect|Ante meridiem|text=For the meridian 180° both east and west of the prime meridian in a geographical coordinate system, see [[Earth:180th meridian|Antimeridian]]}}

The '''12-hour clock''' is a time convention in which the 24&nbsp;hours of the day are divided into two periods: '''a.m.''' (from [[Social:Latin|Latin]] '''{{Langx|la|ante meridiem|label=none}}''', translating to "before midday") and '''p.m.''' (from Latin '''{{Langx|la|post meridiem|label=none}}''', translating to "after midday").<ref name="EncyclopaediaBritannica">{{cite encyclopedia | title =Time | encyclopedia =The New Encyclopædia Britannica | volume =28 | pages =660 2a | year= 1986 }}<br />{{cite encyclopedia | title =Time | encyclopedia = Encyclopædia Britannica. Encyclopædia Britannica Online Library Edition | url = http://library.eb.com.au/eb/article-61027| access-date = 20 November 2013 |quote=The use of AM or PM to designate either noon or midnight can cause ambiguity.}} </ref><ref name=NISTFAQ/> Each period consists of 12&nbsp;hours numbered: 12 (acting as 0),<ref>{{Cite web |url=http://www.math.csusb.edu/faculty/susan/number_bracelets/mod_arith.html |title=Modular Arithmetic |date=25 August 2016 |access-date=28 November 2008 |first=Susan |last=Addington |archive-url=https://web.archive.org/web/20080704121745/http://www.math.csusb.edu/faculty/susan/number_bracelets/mod_arith.html |archive-date=4 July 2008 |url-status=dead }}</ref> 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 [[Place:British Empire|British colonies]], as well as a few other countries. In English-speaking countries: "12&nbsp;p.m." usually indicates noon, while "12&nbsp;a.m." means midnight, but the reverse convention has also been used (see §&nbsp;Confusion at noon and midnight).<ref>{{Cite web |title=Is noon 12 am or 12 pm? |url=https://www.rmg.co.uk/stories/topics/noon-12-am-or-12-pm |access-date=2023-01-26 |website=Royal Museums Greenwich |language=en}}</ref><ref>{{Cite web |title=U.S. Government Printing Office Style Manual - Chapter 12 - Numerals |url=https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2008/html/GPO-STYLEMANUAL-2008-14.htm |access-date=2023-01-27 |website=govinfo.gov}}</ref><ref>{{Cite web |date=2020-10-22 |title=time of day, elapsed time |url=https://www.noslangues-ourlanguages.gc.ca/en/writing-tips-plus/time-of-day-elapsed-time |access-date=2023-01-27 |website=Resources of the Language Portal of Canada}}</ref> "Noon" and "midnight" are unambiguous.

{| align="right" class="wikitable" style="margin: 0 0 2em 2em; font-size: 95%"
|+ Clock system
|-
!24-hour clock !! 12-hour clock
|-
| 00:00||midnight<br />12:00&nbsp;p.m.<br />12:00&nbsp;a.m.<!-- Please leave this as it is. The table is showing what /is/ used, it is not trying to declare what /should/ be used. As the adjacent text explains, there are different conventions in different places.  -->{{Efn|name=noonmidnight}}<br />(start of the day)
|-
| 01:00||1:00&nbsp;a.m.
|-
| 02:00||2:00&nbsp;a.m.
|-
| 03:00||3:00&nbsp;a.m.
|-
| 04:00||4:00&nbsp;a.m.
|-
| 05:00||5:00&nbsp;a.m.
|-
| 06:00||6:00&nbsp;a.m.
|-
| 07:00||7:00&nbsp;a.m.
|-
| 08:00||8:00&nbsp;a.m.
|-
| 09:00||9:00&nbsp;a.m.
|-
| 10:00||10:00&nbsp;a.m.
|-
| 11:00||11:00&nbsp;a.m.
|-
| 12:00||midday<br />noon<br />12:00&nbsp;a.m.<br />12:00&nbsp;p.m.{{Efn|See: {{section link|12-hour clock|Confusion at noon and midnight}}|name=noonmidnight}}
|-
| 13:00||1:00&nbsp;p.m.
|-
| 14:00||2:00&nbsp;p.m.
|-
| 15:00||3:00&nbsp;p.m.
|-
| 16:00||4:00&nbsp;p.m.
|-
| 17:00||5:00&nbsp;p.m.
|-
| 18:00||6:00&nbsp;p.m.
|-
| 19:00||7:00&nbsp;p.m.
|-
| 20:00||8:00&nbsp;p.m.
|-
| 21:00||9:00&nbsp;p.m.
|-
| 22:00||10:00&nbsp;p.m.
|-
| 23:00||11:00&nbsp;p.m.
|-
| 24:00<br><small>(00:00 of next day)</small>||midnight<br />12:00&nbsp;p.m.<br />12:00&nbsp;a.m.<br />(end of the day)
|-
|colspan="3"| {{notelist}}
|}

==History and use==
[[File:Exeter Cathedral astronomical clock.jpg|thumb|upright|[[Exeter Cathedral Astronomical Clock]], showing the double-XII numbering scheme]]
[[File:World War II RAF sector clock.jpg|thumb|World War II RAF [[sector clock]] that can be read either in 12- or 24-hour notation]]

The natural day-and-night division of a calendar day forms the fundamental basis as to why each day is split into two cycles. Originally there were two cycles: 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 eventually evolved into the two 12-hour periods which are used today, one called "a.m." starting at midnight and another called "p.m." starting at noon.<ref name=EncyclopaediaBritannica/>

The 12-hour clock can be traced back as far as [[Place:Mesopotamia|Mesopotamia]] and ancient Egypt.<ref>{{Cite web|url=http://www.arcytech.org/java/clock/clock_history.html|archive-url=https://web.archive.org/web/20081013135550/http://www.arcytech.org/java/clock/clock_history.html|url-status=dead|archive-date=13 October 2008|title=The History of Clocks|date=13 October 2008|access-date=12 November 2017}}</ref> Both an Egyptian [[Engineering:Sundial|sundial]] for daytime use<ref>{{cite web|url=http://members.aon.at/sundials/berlin-egypte.htm|title=Berlin instruments of the old Eg.time of day destination|website=members.aon.at|access-date=13 June 2006|archive-url=https://web.archive.org/web/20061115233642/http://members.aon.at/sundials/berlin-egypte.htm|archive-date=15 November 2006|url-status=live}}</ref> and an Egyptian [[Physics:Water clock|water clock]] for night-time use were found in the tomb of Pharaoh Amenhotep I.<ref>{{cite web|url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks|work=A Walk Through Time|title=Early Clocks|date=12 August 2009 |publisher=[[National Institute of Standards and Technology]]}}</ref> Dating to {{Circa|1500 BC}}, these clocks divided their respective times of use into 12&nbsp;hours each.

The [[History:Ancient Rome|ancient Romans]] also [[Physics:Roman timekeeping|used a 12-hour clock]]: daylight and nighttime were each divided into 12&nbsp;equal intervals (of varying duration according to the season).<ref>{{Cite magazine |last= Bartels |first=Meghan |title=Early Tech Adopters in Ancient Rome Had Portable Sundials |url=https://www.smithsonianmag.com/innovation/early-tech-adopters-ancient-rome-had-portable-sundials-180962225/ |access-date=2025-03-08 |magazine=Smithsonian Magazine |date=February 20, 2017}}</ref> The nighttime hours were grouped into four ''[[Physics:Watchkeeping|watches]]'' (''vigiliae'').<ref>{{cite web |url=https://www.thehourglass.com/story/timekeeping-in-the-roman-army |title=Timekeeping In the Roman army |date=7 July 2018}}</ref>

The first mechanical clocks in the 14th&nbsp;century, if they had dials at all, showed all 24&nbsp;hours using the 24-hour analog dial, influenced by astronomers' familiarity with the [[Astronomy:Astrolabe|astrolabe]] and sundial and by their desire to model the [[Earth's rotation|Earth's apparent motion around the Sun]]. In [[Place:Northern Europe|Northern Europe]] these dials generally used the 12-hour [[Numbering scheme|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, numbering was more likely to be based on the 24-hour system (I to XXIV). The 12-hour clock was used throughout the [[Place:British Empire|British Empire]].

During the 15th and 16th&nbsp;centuries, the 12-hour analog dial and time system 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 [[Astronomy:Astronomical clock|astronomical clock]]s and chronometers.

Most analog clocks and watches today use the 12-hour dial, on which the shorter hour hand rotates once every 12&nbsp;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 also be read in 24-hour notation. This kind of 12-hour clock can be found in countries where the 24-hour clock is preferred.

===Use by country===
[[File:Westclox Big Ben.jpg|thumb|right|Typical [[Clock#Analog|analogue 12-hour clock]]]]
[[File:12 24 Hours World Map.svg|thumb|World map showing the usage of 12 or 24-hour clock in different countries{{legend|#f0027f|24-hour}}{{legend|#990052|24-hour (12-hour orally)}}{{legend|#7fc97f|Both in common use}}{{legend|#386cb0|12-hour (except in special circumstances such as [[aviation]])}}]]
In several countries the 12-hour clock is the dominant written and spoken system of time, predominantly in nations that were part of the former British Empire, for example, the United Kingdom, Republic of Ireland, the [[United States]], Canada (excluding Quebec), Australia, [[Earth:New Zealand|New Zealand]], South Africa, [[Place:India|India]], Pakistan, and Bangladesh, and others follow this convention as well, such as Mexico, Colombia, and the former American colony of the Philippines. Even in those countries where the 12-hour clock is predominant, there are frequently contexts (such as science, medicine, the military or transport) in which the 24-hour clock is preferred. In most countries, however, the [[Physics:24-hour clock|24-hour clock]] is the standard system used, especially in writing. Some nations in Europe and Latin America use a combination of the two, preferring the 12-hour system in colloquial speech but using the 24-hour system in written form and in formal contexts.

The 12-hour clock in speech often uses phrases such as'' ...&nbsp;in the morning'','' ...&nbsp;in the afternoon'','' ...&nbsp;in the evening'', and ''...&nbsp;at night''. ''Rider's British Merlin'' almanac for 1795 and a similar almanac for 1773 published in London used them.<ref>{{cite book|url=http://catalogue.nla.gov.au/Record/4881611|title=National Library of Australia catalogue entry for ''Rider's British merlin: for the year of Our Lord God 1795''|year=1795 |publisher=Printed for the Company of Stationers, and sold by R. Horsfield, at Stationers-hall, near Ludgate-street |archive-url=https://web.archive.org/web/20130522172644/http://catalogue.nla.gov.au/Record/4881611|archive-date=22 May 2013|url-status=live}}</ref> Other than in English-speaking countries and some Spanish-speaking countries, the terms ''a.m.'' and ''p.m.'' are seldom used and often unknown.{{efn-lg|name=Other}}

===Computer support===
In most countries, computers by default show the time in 24-hour notation. Most operating systems, including [[Software:Microsoft Windows|Microsoft Windows]] and [[Unix-like]] systems such as [[Software:Linux|Linux]] and [[Software:MacOS|macOS]], activate the 12-hour notation by default for a limited number of language and region settings. This behaviour can be changed by the user, such as with the [[Software:Microsoft Windows|Windows]] operating system's "Region and Language" settings.<ref>{{cite web|url=http://www.bleepingcomputer.com/tutorials/customize-how-time-is-displayed-in-windows/|title=How to customize how the time is displayed in Windows|author=Lawrence Abrams|date=13 December 2012|access-date=26 October 2013|publisher=Bleeping Computer|archive-url=https://web.archive.org/web/20131029183705/http://www.bleepingcomputer.com/tutorials/customize-how-time-is-displayed-in-windows/|archive-date=29 October 2013|url-status=live}}</ref>

==Abbreviations==

[[File:Digital-clock-alarm.jpg|thumb|Typical [[digital clock|digital]] 12-hour alarm clock indicating ''p.m.'' with a dot to the left of the hour.]]The [[Social:Latin|Latin]] abbreviations ''a.m.'' and ''p.m.'' (often written "am" and "pm", "AM" and "PM", or "A.M." and "P.M.") are used in [[Social:English language|English]] (and [[Spanish language|Spanish]]).<ref>{{cite dictionary|dictionary=Diccionario panhispánico de dudas|url=http://buscon.rae.es/dpdI/SrvltGUIBusDPD?lema=hora2|title=hora|archive-url=https://web.archive.org/web/20110720024649/http://buscon.rae.es/dpdI/SrvltGUIBusDPD?lema=hora2|archive-date=20 July 2011|url-status=live|language=Spanish}}</ref>{{efn-lg|name=Other|The equivalents in [[Greek language|Greek]] are {{lang|el|π.μ.}} and {{lang|el|μ.μ.}}, respectively, and in [[Social:Sinhala language|Sinhala]] {{lang|si|පෙ.ව.}} ({{transliteration|si|''pe.va.''}}) for {{lang|si|පෙරවරු}} ({{transliteration|si|''peravaru''}}, {{lang|si|පෙර}} {{transliteration|si|''pera''}} – fore, pre) and {{lang|si|ප.ව.}} ({{transliteration|si|''pa.va.''}}) for {{lang|si|පස්වරු}} ({{transliteration|si|''pasvaru''}}, {{lang|si|පස්සේ}} {{transliteration|si|''passē''}}&nbsp;– after, post). In [[Social:Irish language|Irish]], ''a.m.'' and ''i.n.'' are used, standing for ''ar maidin'' ("in the morning") and ''iarnóin'' ("afternoon") respectively. In Portuguese, there are two official options and many others used, for example, using 21:45, 21h45 or 21h45min (official ones) or 21:45 or 9:45&nbsp;p.m. Most other languages lack formal abbreviations for "before noon" and "after noon", and their users use the 12-hour clock only orally and informally. In those languages, such as Polish, Russian, and Hebrew, 24-hour clock is always used in writing, even informal writing, though 12-hour designations may be used colloquially in speech, such as "9 in the morning" or "3 in the night".<ref>{{cite web |title=Jak poprawnie zapisywać daty i inne określenia czasu? |trans-title=How do I correctly write dates and other time expressions? |url=https://synomix.pl/jak-poprawnie-zapisywac-daty-i-inne-okreslenia-czasu |access-date=18 March 2025 |language=Polish}}</ref> In those languages, an email saying "let's meet at 15:00" might be read out aloud as "let's meet at three in the afternoon".}} 'Noon' is not abbreviated. 
When abbreviations and phrases are omitted, one may rely on sentence context and societal norms to reduce ambiguity. For example, if one commutes to work at "9:00", 9:00&nbsp;a.m. may be implied, but if a social dance is scheduled to begin at "9:00", it may begin at 9:00&nbsp;p.m.

==Related conventions==

===Typography===
The terms "a.m." and "p.m." are abbreviations of the Latin {{Lang|la|ante meridiem}} (before midday) and {{Lang|la|post meridiem}} (after midday). Depending on the [[Style guide|style guide]] referenced, the abbreviations "a.m." and "p.m." are variously written in small capitals ("{{smallcaps|am}}" and "{{smallcaps|pm}}"),<ref>{{cite web |last1=Trask |first1=Larry |title=Small Capitals |url=https://www.sussex.ac.uk/informatics/punctuation/misc/small |website=University of Sussex Informatics |publisher=The University of Sussex |access-date=30 October 2024}}</ref><ref name="Chicago MOS">{{cite web |title=9.39: Numerals versus words for time of day |url=https://www.chicagomanualofstyle.org/book/ed18/part2/ch09/psec039.html |website=The Chicago Manual of Style Online |publisher=The University of Chicago |access-date=30 October 2024}}</ref> [[Social:Letter case|uppercase]] letters without a period ("AM" and "PM"), uppercase letters with periods, or lowercase letters ("am" and "pm"<ref name=ESG /> or "a.m." and "p.m."<ref name="Chicago MOS"/>). With the advent of computer generated and printed schedules, especially airlines, advertising, and television promotions, the "M" character is often omitted as providing no additional information as in "9:30A" or "10:00P".<ref>{{cite web |last=Watson |first=James Robert |title=A more efficient way to denote am or pm |url=https://www.jamesrobertwatson.com/ampm.html |website=Jim Watson professor of design |access-date=August 30, 2022}}</ref>

Some style guides 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.<ref>Hacker, Diana, A Writer's Reference, six edition, Bedford, St Martin's, Boston, 2007, section M4-c, p.308.</ref>

The hour/minute separator varies between countries: some use a colon, others use a period (full stop),<ref name=ESG>{{cite book |title=Economist Style Guide |page=185 |edition=12th |isbn=9781781258316 |publisher=[[Finance:The Economist|The Economist]] |date=2018}}</ref> and still others use the letter h. <!-- Dubious. I have seen the h notation many times but only ever with 24-hour clock. --> (In some usages, particularly "military time", of the [[Physics:24-hour clock|24-hour clock]], there is no separator between hours and minutes.<ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |date=May 27, 2015 |title=The Tongue and Quill |publisher=US Air Force |url=https://static.e-publishing.af.mil/production/1/saf_cn/publication/afh33-337/afh33-337.pdf |page=356 |access-date=26 October 2021 |archive-date=26 November 2020 |archive-url=https://web.archive.org/web/20201126183321/https://static.e-publishing.af.mil/production/1/saf_cn/publication/afh33-337/afh33-337.pdf |url-status=dead }} Air Force Handbook 33-337.</ref> This style is not generally seen when the 12-hour clock is used.)

=== Encoding ===
[[Unicode]] specifies codepoints for ''a.m.'' and ''p.m.'' as [[Precomposed character|precomposed character]]s, which are intended to be used only with Chinese-Japanese-Korean (CJK) character sets, as they take up exactly the same space as one CJK character: 
* {{unichar|33C2}}
* {{unichar|33D8}}

===Informal speech and rounding off===
In speaking, it is common to round the time to the nearest five&nbsp;minutes and/or express the time as the past (or to) the closest hour; for example, "five past five" (5:05). Minutes ''past'' the hour means those minutes are added to the hour; "ten past five" means 5:10. Minutes ''to, till'' and ''of'' the hour mean those minutes are subtracted; "ten of five", "ten till five", and "ten to five" all mean 4:50.

Fifteen&nbsp;minutes is often called a ''quarter hour'', and thirty&nbsp;minutes is often known as a ''half hour''. For example, 5:15 can be phrased "(a) quarter past five" or "five-fifteen"; 5:30 can be "half past five", "five-thirty" or simply "half five". The time 8:45 may be spoken as "eight&nbsp;forty-five" or "(a) quarter to nine".<ref>{{cite dictionary|title=quarter|dictionary=American Heritage Dictionary|edition=3rd|location=Boston|publisher=Houghton Mifflin|date=1992}} s.v. usage note at end of "quarter" entry.</ref> In some languages, e.g. Polish, rounding off is mandatory when using (spoken) 12-hour clock, but disallowed when using 24 hour notation. I.e. 15:12 might be pronounced as "quarter past three" or "fifteen-twelve", but ''not'' "three-twelve" or "quarter past fifteen".<ref>{{cite web |title=Określanie godzin i minut |url=https://sjp.pwn.pl/poradnia/haslo/Okreslanie-godzin-i-minut;19024.html |website=Słownik języka polskiego PWN |access-date=18 March 2025 |language=Polish}}</ref>

In older English, it was common for the number 25 to be expressed as "five-and-twenty".<ref>{{cite book |last1=Dickens |first1=Charles |title=Little Dorrit |date=1855 |page=Chapter 27}}</ref> In this way the time 8:35 might have been phrased as "five-and-twenty to 9",<ref>{{cite web |last1=Trudgill |first1=Peter |title=Number five-and-twenty: A fading linguistic practice |url=https://www.theneweuropean.co.uk/brexit-news-peter-trudgill-recounts-the-old-way-of-counting-80170/ |website=The New European |date=17 May 2020 |access-date=13 October 2022}}</ref> although this styling fell out of fashion in the later part of the 1900s and is now rarely used.<ref>{{cite web |last1=Swan |first1=Michael |title=Ask About English |url=https://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1535_questionanswer/page63.shtml |website=BBC World Service |publisher=BBC |access-date=13 January 2021}}</ref>

Instead of meaning 5:30, the "half five" expression is sometimes used to mean 4:30, or "halfway to five", especially for regions such as the American Midwest and other areas that have been particularly influenced by German culture.{{cn|date=November 2023}} This meaning follows the pattern choices of many Germanic and [[Social:Slavic languages|Slavic languages]], including Serbo-Croatian, [[Dutch language|Dutch]], [[Danish language|Danish]], [[Russian language|Russian]], [[Social:Norwegian language|Norwegian]], and [[Swedish language|Swedish]], as well as [[Social:Hungarian language|Hungarian]], [[Social:Finnish language|Finnish]], and the languages of the [[Place:Baltic states|Baltic States]].

Moreover, in situations where the relevant hour is obvious or has been recently mentioned, a speaker might omit the hour and just say "quarter to (the hour)", "half past" or "ten till" to avoid an elaborate sentence in informal conversations. These forms are often commonly used in television and radio broadcasts that cover multiple time zones at one-hour intervals.<ref>{{Cite magazine|magazine=TVTimes|date=21–27 May 1983|url=http://forums.doyouremember.co.uk/threads/8061-TVTimes-magazine-21-27-May-1983-part1 |archive-url=https://web.archive.org/web/20121018072904/http://forums.doyouremember.co.uk/threads/8061-TVTimes-magazine-21-27-May-1983-part1|archive-date=18 October 2012|url-status=live |title=TVTimes magazine 21-27 May 1983 part1 |access-date=8 October 2019}}</ref>

In describing a vague time of day, a speaker might say the phrase "seven-thirty, eight" to mean sometime around 7:30 or 8:00. Such phrasing can be misinterpreted for a specific time of day (here 7:38), especially by a listener not expecting an estimation. The phrase "''about'' seven-thirty ''or'' eight" clarifies this.

Some more ambiguous phrasing might be avoided. Within five&nbsp;minutes of the hour, the phrase "five of seven" (6:55) can be heard "five-oh-seven" (5:07). "Five ''to'' seven" or even "six&nbsp;fifty-five" clarifies this.

===Formal speech and times to the minute===
{{redirect|O'clock|the o'clock watch|O bag}}
Minutes may be expressed as an exact number of minutes past the hour specifying the time of day (e.g., 6:32&nbsp;p.m. is "six&nbsp;thirty-two"). Additionally, when expressing the time using the "past (after)" or "to (before)" formula, it is conventional to choose the number of minutes below 30 (e.g., 6:32&nbsp;p.m. is conventionally "twenty-eight&nbsp;minutes to seven" rather than "thirty-two&nbsp;minutes past six").

In spoken English, full hours are often represented by 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 some 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&nbsp;a.m. can be pronounced ''six&nbsp;oh&nbsp;two&nbsp;a.m.'' whereas 6:32&nbsp;a.m. could be told as ''six&nbsp;thirty-two&nbsp;a.m.''.

==Confusion at noon and midnight==
{| align="right" class="wikitable" style="margin: 0 0 1em 1em; font-size: 95%; width: 35em"
|+ Time according to various conventions
|-
!Device or [[Style guide|style]]||Midnight<br /><small>Start of day</small>||[[Astronomy:Noon|Noon]]||Midnight<br /><small>End of day</small>
|-
|Written 24-hour time<ref>{{cite book|title = Chicago Manual of Style | at = paragraph 9.41 | edition = 18th | date = 2024 | publisher = University of Chicago Press | isbn=978-0-226-81797-2}}</ref> ||00:00||12:00||24:00 or 00:00 of following day
|-
|U.S. Government Publishing Office (1953)<ref name="GPO1953">{{cite web |url=https://www.govinfo.gov/content/pkg/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f/pdf/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f.pdf |pages=152, 267 |title=United States Government Printing Office Style Manual |date=January 1953 |website=govinfo |publisher=U.S. Government Publishing Office |access-date=5 September 2019 |archive-url=https://web.archive.org/web/20190905161607/https://www.govinfo.gov/content/pkg/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f/pdf/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f.pdf |archive-date=5 September 2019 |url-status=live }}</ref>||12&nbsp;p.m.{{efn|name=startend|These styles are ambiguous with respect to whether midnight is at the start or end of each day.}}||12&nbsp;m.||12&nbsp;p.m.{{efn|name=startend}}
|-
|U.S. Government Publishing Office (2000)<ref name="GPO2000">{{cite web |url=https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2000/pdf/GPO-STYLEMANUAL-2000.pdf |title=U.S. Government Printing Office Style Manual |at=page 156 |website=govinfo |date=2000 |publisher=U.S. Government Publishing Office |access-date=5 September 2019 |archive-url=https://web.archive.org/web/20190905160112/https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2000/pdf/GPO-STYLEMANUAL-2000.pdf |archive-date=5 September 2019 |url-status=live }}</ref>||12&nbsp;p.m.{{efn|name=startend}}||12&nbsp;a.m.||12&nbsp;p.m.{{efn|name=startend}}
|-
|U.S. Government Publishing Office (2008)<ref name="GPO2008">{{cite web|url=https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2008/pdf/GPO-STYLEMANUAL-2008.pdf|title=U.S. Government Printing Office Style Manual|page=271|website=govinfo|date=2008|publisher=U.S. Government Publishing Office|access-date=5 September 2019|archive-url=https://web.archive.org/web/20190412235057/https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2008/pdf/GPO-STYLEMANUAL-2008.pdf|archive-date=12 April 2019|url-status=live}}</ref>||12&nbsp;a.m.{{efn|name=startend}}||12&nbsp;p.m.||12&nbsp;a.m.{{efn|name=startend}}
|-
|Japanese legal convention<ref>{{cite web |script-title=ja:午前１２時？ 午後０時？ |trans-title=12&nbsp;AM? or 0&nbsp;PM? |url=http://jjy.nict.go.jp/QandA/FAQ/12am-or-0pm-J.html |date=15 February 1989 |website=[[Organization:National Institute of Information and Communications Technology|National Institute of Information and Communications Technology]] |language=ja |access-date=24 May 2017 |archive-url=https://web.archive.org/web/20170606032129/http://jjy.nict.go.jp/QandA/FAQ/12am-or-0pm-J.html |archive-date=6 June 2017 |url-status=dead}}</ref>||{{lang|ja|午前0時}}<br />(0&nbsp;a.m.)
|{{lang|ja|午前12時}}<br />(12&nbsp;a.m.)
|{{lang|ja|午後12時}}<br />(12&nbsp;p.m.)
|-
|Japanese common usage<ref name="NAO Japan">{{cite news |script-title=ja:質問4-1）正午は午前12時？それとも、午後12時？ |trans-title=Question 4-1) Is noon 12&nbsp;a.m.? Or 12&nbsp;p.m.? |url=https://www.nao.ac.jp/faq/a0401.html |website=[[Organization:National Astronomical Observatory of Japan|National Astronomical Observatory of Japan]] |language=ja |access-date=19 January 2022  }}</ref>||{{lang|ja|午前0時}}<br />(0&nbsp;a.m.)
|{{lang|ja|午後0時}}<br />(0&nbsp;p.m.)
|{{lang|ja|午後12時}}<br />(12&nbsp;p.m.)
|-
|Canadian Press,<ref name="CP" /> UK standard<ref name="NPL" />||midnight||noon||midnight
|-
|Associated Press style<ref name="AP">Paula Froke, Anna Joe Bratton, Oskar Garcia, Jeff McMillan & Jerry Schwart, Eds., 54th ed., ''The Associated Press Stylebook and Briefing on Media Law'', New York: Basic Books, June 2019, {{ISBN|978-1-5416-9989-2}}, s.v. noon, midnight, times.</ref>||—||noon||midnight
|-
|[[National Institute of Standards and Technology|NIST]]<ref name="NISTFAQ" />||midnight{{efn|name=niststartend|NIST recommends using 11:59&nbsp;p.m. and 12:01&nbsp;a.m. to disambiguate when needed.}}<br />12:01&nbsp;a.m.||noon||midnight{{efn|name=niststartend}}<br />11:59&nbsp;p.m.
|-
|colspan="4"|{{noteslist}}
|}

It is not always clear what times "12:00&nbsp;a.m." and "12:00&nbsp;p.m." denote. In [[Social:Latin|Latin]], ''{{Lang|la|ante meridiem}}'' (a.m.) means "before midday" and ''{{Lang|la|post meridiem}}'' (p.m.) means "after midday". Since noon is neither before nor after itself, the terms a.m. and p.m. do not apply.<ref name=NISTFAQ /> Although noon could be denoted "12&nbsp;m.", this is seldom done<ref name="chicago">{{cite book|title = Chicago Manual of Style | at = paragraph 9.40 | edition = 18th | date = 2024 | publisher = University of Chicago Press | quote = Although noon can be expressed as 12:00 m. (m = ''meridies''), very few use that form. | isbn=978-0-226-81797-2}}</ref> and also does not resolve the question of how to indicate midnight.

By convention, "12&nbsp;a.m." denotes midnight and "12&nbsp;p.m." denotes noon.<ref name="AHDEL">{{cite dictionary|url=http://ahdictionary.com/word/search.html?q=AM|title=AM|archive-url=https://web.archive.org/web/20140109230904/http://ahdictionary.com/word/search.html?q=AM|archive-date=9 January 2014|url-status=live|dictionary=The American Heritage Dictionary of the English Language|edition=Fifth|date=2011}}</ref>
However, many style guides recommend against using either because of the potential for confusion. Many recommend instead using the unambiguous terms "12&nbsp;noon" and "12&nbsp;midnight", or simply "noon" and "midnight". These include ''The American Heritage Dictionary of the English Language'',<ref name="AHDEL" />
''The Canadian Press Stylebook'',<ref name="CP">{{cite book|title=The Canadian Press Stylebook|edition= 11th |date = 1999 | at = page 288}}</ref>
and the [[National Institute of Standards and Technology|NIST]]'s "Frequently asked questions (FAQ)" web page.<ref name=NISTFAQ>{{cite web|url=https://www.nist.gov/pml/time-and-frequency-division/times-day-faqs|title=Times of Day FAQs |publisher=[[National Institute of Standards and Technology]]|date=21 September 2016|access-date=30 September 2018|archive-url=https://web.archive.org/web/20180921153333/https://www.nist.gov/pml/time-and-frequency-division/times-day-faqs|archive-date=21 September 2018|url-status=live}}</ref>

Alternatively, some recommend referring to one minute before or after 12:00, especially when referring to midnight (for example, "11:59&nbsp;p.m." or "12:01&nbsp;a.m."). These include the UK's [[Organization:National Physical Laboratory (United Kingdom)|National Physical Laboratory]] "FAQ-Time" web page.<ref name="NPL">{{Cite web |url=http://www.npl.co.uk/science-technology/time-frequency/time/faqs |title=National Physical Laboratory, ''FAQ-Time'' |access-date=11 January 2015 |archive-url=https://web.archive.org/web/20150303095129/http://www.npl.co.uk/science-technology/time-frequency/time/faqs/ |archive-date=3 March 2015 |url-status=dead }}</ref>
That has become common in the United States in legal contracts and for [[Engineering:Airplane|airplane]], bus, or [[Engineering:Train|train]] schedules, though some schedules use other conventions. Occasionally, when trains run at regular intervals, the pattern may be broken at midnight by displacing the midnight departure one or more minutes, such as to 11:59&nbsp;p.m. or 12:01&nbsp;a.m.<ref>{{cite web|url=http://www.abelliogreateranglia.co.uk/f/62130/62130.pdf|title=Cheshunt/Enfield Town to London Liverpool Street via Seven Sisters service Interim train timetables|archive-url=https://web.archive.org/web/20150526154117/http://www.abelliogreateranglia.co.uk/f/62130/62130.pdf|archive-date=26 May 2015|url-status=dead|website=Abellio Greater Anglia London|date=17 May 2015|pages=7,8}}</ref>

Some authors have been known to use the reverse of the normal convention. E. G. Richards in his book ''Mapping Time'' (1999) provided a diagram in which 12&nbsp;a.m. means noon and 12&nbsp;p.m. means midnight.<ref>{{cite book|last=Richards|first=E. G.|title=Mapping Time: the Calendar and its History|publisher=[[Organization:Oxford University Press|Oxford University Press]]|date=1999|page=289}}</ref>
Historically, the style manual of the United States Government Printing Office used 12&nbsp;a.m. for noon and 12&nbsp;p.m. for midnight, though this was reversed in its 2008 editions.<ref name="GPO2000" /><ref name="GPO2008" />

In Japanese usage, midnight is written as {{lang|ja|午前0時}} (0&nbsp;a.m.) and noon is written as {{lang|ja|午後0時}} (0&nbsp;p.m.), making the hours numbered sequentially from 0 to 11 in both halves of the day. Alternatively, noon may be written as {{lang|ja|午前12時}} (12&nbsp;a.m.) and midnight at the end of the day as {{lang|ja|午後12時}} (12&nbsp;p.m.), as opposed to {{lang|ja|午前0時}} (0&nbsp;a.m.) for the start of the day, making the Japanese convention the opposite of the English usage of 12&nbsp;a.m. and 12&nbsp;p.m.<ref name="NAO Japan" />

==See also==
{{div col|colwidth=30em}}
* [[Physics:24-hour clock|24-hour clock]]
* [[Clock position]]
* Date and time representation by country
* [[Decimal time]]
* [[Physics:Italian six-hour clock|Italian six-hour clock]]
* Midnight
* [[Muhurta]]
* [[Astronomy:Noon|Noon]]
* [[Physics:Pahar|Pahar]]
* [[Physics:Thai six-hour clock|Thai six-hour clock]]
{{div col end}}

==Notes==
{{notelist-lg}} <!-- This style has to be used because the 'ordinary' template:efn is being used in the tables. -->
==References==
{{reflist|30em}}

==External links==
* [https://www.nist.gov/pml/div688/times.cfm NIST FAQ on time] {{Webarchive|url=https://web.archive.org/web/20120916120545/http://www.nist.gov/pml/div688/times.cfm |date=16 September 2012 }}
* [https://longtailworld.blogspot.com/2006/04/japan1212am-is-noon-in-japan.html 12am is noon in Japan]
{{Time measurement and standards}}

{{DEFAULTSORT:12-Hour Clock}}
[[Category:Time measurement systems]]

{{Sourceattribution|12-hour clock}}