12-hour clock

Time counting system AM and PM other uses AM PM (disambiguation) Ante meridiem For the meridian 180° both east and west of the prime meridian in a geographical coordinate system, see Antimeridian January 2021 November 2017 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 la ante meridiem none, translating to "before midday") and p.m. (from Latin la post meridiem none, translating to "after midday").Time The New Encyclopædia Britannica 28 660 2a 1986 Time Encyclopædia Britannica. Encyclopædia Britannica Online Library Edition http://library.eb.com.au/eb/article-61027 20 November 2013 The use of AM or PM to designate either noon or midnight can cause ambiguity. Each period consists of 12 hours numbered: 12 (acting as 0),http://www.math.csusb.edu/faculty/susan/number_bracelets/mod_arith.html Modular Arithmetic 25 August 2016 28 November 2008 Susan Addington https://web.archive.org/web/20080704121745/http://www.math.csusb.edu/faculty/susan/number_bracelets/mod_arith.html 4 July 2008 dead 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. In English-speaking countries: "12 p.m." usually indicates noon, while "12 a.m." means midnight, but the reverse convention has also been used (see § Confusion at noon and midnight).Is noon 12 am or 12 pm? https://www.rmg.co.uk/stories/topics/noon-12-am-or-12-pm 2023-01-26 Royal Museums Greenwich enU.S. Government Printing Office Style Manual - Chapter 12 - Numerals https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2008/html/GPO-STYLEMANUAL-2008-14.htm 2023-01-27 govinfo.gov2020-10-22 time of day, elapsed time https://www.noslangues-ourlanguages.gc.ca/en/writing-tips-plus/time-of-day-elapsed-time 2023-01-27 Resources of the Language Portal of Canada "Noon" and "midnight" are unambiguous. + Notation according to various authorities24-hour clock 12-hour clock 00:00midnight12:00 p.m.noonmidnight(start of the day)12:00 a.m.noonmidnight(start of the day) 01:001:00 a.m. 02:002:00 a.m. 03:003:00 a.m. 04:004:00 a.m. 05:005:00 a.m. 06:006:00 a.m. 07:007:00 a.m. 08:008:00 a.m. 09:009:00 a.m. 10:0010:00 a.m. 11:0011:00 a.m. 12:00middaynoon12:00 a.m.noonmidnight(start of the day)12:00 p.m.See: 12-hour clock Confusion at noon and midnight noonmidnight 13:001:00 p.m. 14:002:00 p.m. 15:003:00 p.m. 16:004:00 p.m. 17:005:00 p.m. 18:006:00 p.m. 19:007:00 p.m. 20:008:00 p.m. 21:009:00 p.m. 22:0010:00 p.m. 23:0011:00 p.m. 24:00(00:00 of next day)midnight12:00 p.m.noonmidnight(start of the day)12:00 a.m.noonmidnight(start of the day)(end of the day) History and use thumb|upright|Exeter Cathedral Astronomical Clock, showing the double-XII numbering scheme 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. The 12-hour clock can be traced back as far as Mesopotamia and ancient Egypt.http://www.arcytech.org/java/clock/clock_history.html https://web.archive.org/web/20081013135550/http://www.arcytech.org/java/clock/clock_history.html dead 13 October 2008 The History of Clocks 13 October 2008 12 November 2017 Both an Egyptian sundial for daytime usehttp://members.aon.at/sundials/berlin-egypte.htm Berlin instruments of the old Eg.time of day destination members.aon.at 13 June 2006 https://web.archive.org/web/20061115233642/http://members.aon.at/sundials/berlin-egypte.htm 15 November 2006 live and an Egyptian water clock for night-time use were found in the tomb of Pharaoh Amenhotep I.https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks A Walk Through Time Early Clocks 12 August 2009 National Institute of Standards and Technology Dating to 1500 BC, these clocks divided their respective times of use into 12 hours each. The ancient Romans also used a 12-hour clock: daylight and nighttime were each divided into 12 equal intervals (of varying duration according to the season). Bartels Meghan Early Tech Adopters in Ancient Rome Had Portable Sundials https://www.smithsonianmag.com/innovation/early-tech-adopters-ancient-rome-had-portable-sundials-180962225/ 2025-03-08 Smithsonian Magazine February 20, 2017 The nighttime hours were grouped into four watches (vigiliae).https://www.thehourglass.com/story/timekeeping-in-the-roman-army Timekeeping In the Roman army 7 July 2018 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 by their desire to model the Earth's apparent motion around 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, numbering was more likely to be based on the 24-hour system (I to XXIV). The 12-hour clock was used throughout the British Empire. During the 15th and 16th 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 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 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 thumb|World map showing the usage of 12 or 24-hour clock in different countries#f0027f 24-hour written#7fc97f Both written by different groups#386cb0 12-hour written (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, New Zealand, South Africa, 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 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 ... in the morning, ... in the afternoon, ... in the evening, and ... at night. Rider's British Merlin almanac for 1795 and a similar almanac for 1773 published in London used them.http://catalogue.nla.gov.au/Record/4881611 National Library of Australia catalogue entry for Rider's British merlin: for the year of Our Lord God 1795 1795 Printed for the Company of Stationers, and sold by R. Horsfield, at Stationers-hall, near Ludgate-street https://web.archive.org/web/20130522172644/http://catalogue.nla.gov.au/Record/4881611 22 May 2013 live Other than in English-speaking countries and some Spanish-speaking countries, the terms a.m. and p.m. are seldom used and often unknown.Other Computer support In most countries, computers by default show the time in 24-hour notation. Most operating systems, including Microsoft Windows and Unix-like systems such as Linux and 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 Windows operating system's "Region and Language" settings.http://www.bleepingcomputer.com/tutorials/customize-how-time-is-displayed-in-windows/ How to customize how the time is displayed in Windows Lawrence Abrams 13 December 2012 26 October 2013 Bleeping Computer https://web.archive.org/web/20131029183705/http://www.bleepingcomputer.com/tutorials/customize-how-time-is-displayed-in-windows/ 29 October 2013 live Abbreviations Digital-clock-alarm.jpg Typical digital 12-hour alarm clock indicating p.m. with a dot to the left of the hour. 300 Westclox Big Ben.jpg Typical analogue 12-hour clock 200 Date and time representation by country 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).Diccionario panhispánico de dudas http://buscon.rae.es/dpdI/SrvltGUIBusDPD?lema=hora2 hora https://web.archive.org/web/20110720024649/http://buscon.rae.es/dpdI/SrvltGUIBusDPD?lema=hora2 20 July 2011 live SpanishOther The equivalents in Greek are el π.μ. and el μ.μ., respectively, and in Sinhala si පෙ.ව. (si pe.va.) for si පෙරවරු (si peravaru, si පෙර si pera – fore, pre) and si ප.ව. (si pa.va.) for si පස්වරු (si pasvaru, si පස්සේ si passē – after, post). In 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 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.December 2007 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".Jak poprawnie zapisywać daty i inne określenia czasu? How do I correctly write dates and other time expressions? https://synomix.pl/jak-poprawnie-zapisywac-daty-i-inne-okreslenia-czasu 18 March 2025 Polish 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 a.m. may be implied, but if a social dance is scheduled to begin at "9:00", it may begin at 9:00 p.m. Related conventions Typography The terms "a.m." and "p.m." are abbreviations of the Latin la ante meridiem (before midday) and la post meridiem (after midday). Depending on the style guide referenced, the abbreviations "a.m." and "p.m." are variously written in small capitals ("am" and "pm"),Trask Larry Small Capitals https://www.sussex.ac.uk/informatics/punctuation/misc/small University of Sussex Informatics The University of Sussex 30 October 20249.39: Numerals versus words for time of day https://www.chicagomanualofstyle.org/book/ed18/part2/ch09/psec039.html The Chicago Manual of Style Online The University of Chicago 30 October 2024 uppercase letters without a period ("AM" and "PM"), uppercase letters with periods, or lowercase letters ("am" and "pm" or "a.m." and "p.m."). Some style guides suggest the use of a space between the number and the a.m. or p.m. abbreviation.March 2011 Style guides recommend not using a.m. and p.m. without a time preceding it.Hacker, Diana, A Writer's Reference, six edition, Bedford, St Martin's, Boston, 2007, section M4-c, p.308. The hour/minute separator varies between countries: some use a colon, others use a period (full stop),Economist Style Guide 185 12th 9781781258316 The Economist 2018 and still others use the letter h.October 2021 (In some usages, particularly "military time", of the 24-hour clock, there is no separator between hours and minutes. May 27, 2015 The Tongue and Quill US Air Force https://static.e-publishing.af.mil/production/1/saf_cn/publication/afh33-337/afh33-337.pdf 356 26 October 2021 26 November 2020 https://web.archive.org/web/20201126183321/https://static.e-publishing.af.mil/production/1/saf_cn/publication/afh33-337/afh33-337.pdf dead Air Force Handbook 33-337. 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 characters, 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: 33C2 33D8 Informal speech and rounding off In speaking, it is common to round the time to the nearest five 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 minutes is often called a quarter hour, and thirty 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 forty-five" or "(a) quarter to nine".quarter American Heritage Dictionary 3rd Boston Houghton Mifflin 1992 s.v. usage note at end of "quarter" entry. 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".Określanie godzin i minut https://sjp.pwn.pl/poradnia/haslo/Okreslanie-godzin-i-minut;19024.html Słownik języka polskiego PWN 18 March 2025 Polish In older English, it was common for the number 25 to be expressed as "five-and-twenty".Dickens Charles Little Dorrit 1855 Chapter 27 In this way the time 8:35 might have been phrased as "five-and-twenty to 9",Trudgill Peter Number five-and-twenty: A fading linguistic practice https://www.theneweuropean.co.uk/brexit-news-peter-trudgill-recounts-the-old-way-of-counting-80170/ The New European 17 May 2020 13 October 2022 although this styling fell out of fashion in the later part of the 1900s and is now rarely used.Swan Michael Ask About English https://www.bbc.co.uk/worldservice/learningenglish/radio/specials/1535_questionanswer/page63.shtml BBC World Service BBC 13 January 2021 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.November 2023 This meaning follows the pattern choices of many Germanic and Slavic languages, including Serbo-Croatian, Dutch, Danish, Russian, Norwegian, and Swedish, as well as Hungarian, Finnish, and the languages of the 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.TVTimes 21–27 May 1983 http://forums.doyouremember.co.uk/threads/8061-TVTimes-magazine-21-27-May-1983-part1 https://web.archive.org/web/20121018072904/http://forums.doyouremember.co.uk/threads/8061-TVTimes-magazine-21-27-May-1983-part1 18 October 2012 live TVTimes magazine 21-27 May 1983 part1 8 October 2019 Formal speech and times to the minute O'clock the o'clock watch O bag October 2020 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 "six 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 p.m. is conventionally "twenty-eight minutes to seven" rather than "thirty-two 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 a.m. can be pronounced six oh two a.m. whereas 6:32 a.m. could be told as six thirty-two a.m.. Confusion at noon and midnight + Time according to various conventionsDevice or styleMidnightStart of dayNoonMidnightEnd of dayWritten 24-hour time Chicago Manual of Style paragraph 9.41 18th 2024 University of Chicago Press 978-0-226-81797-2 00:0012:0024:00 or 00:00 of following dayU.S. Government Publishing Office (1953)https://www.govinfo.gov/content/pkg/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f/pdf/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f.pdf 152, 267 United States Government Printing Office Style Manual January 1953 govinfo U.S. Government Publishing Office 5 September 2019 https://web.archive.org/web/20190905161607/https://www.govinfo.gov/content/pkg/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f/pdf/GOVPUB-GP-6eda8715b8f5cb1d8514325b97334d4f.pdf 5 September 2019 live 12 p.m.startend These styles are ambiguous with respect to whether midnight is at the start or end of each day.12 m.12 p.m.startendU.S. Government Publishing Office (2000)https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2000/pdf/GPO-STYLEMANUAL-2000.pdf U.S. Government Printing Office Style Manual page 156 govinfo 2000 U.S. Government Publishing Office 5 September 2019 https://web.archive.org/web/20190905160112/https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2000/pdf/GPO-STYLEMANUAL-2000.pdf 5 September 2019 live 12 p.m.startend12 a.m.12 p.m.startendU.S. Government Publishing Office (2008)https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2008/pdf/GPO-STYLEMANUAL-2008.pdf U.S. Government Printing Office Style Manual 271 govinfo 2008 U.S. Government Publishing Office 5 September 2019 https://web.archive.org/web/20190412235057/https://www.govinfo.gov/content/pkg/GPO-STYLEMANUAL-2008/pdf/GPO-STYLEMANUAL-2008.pdf 12 April 2019 live12 a.m.startend12 p.m.12 a.m.startendJapanese legal conventionja:午前１２時？ 午後０時？ 12 AM? or 0 PM? http://jjy.nict.go.jp/QandA/FAQ/12am-or-0pm-J.html 15 February 1989 National Institute of Information and Communications Technology ja 24 May 2017 https://web.archive.org/web/20170606032129/http://jjy.nict.go.jp/QandA/FAQ/12am-or-0pm-J.html 6 June 2017 deadja 午前0時(0 a.m.)ja 午前12時(12 a.m.)ja 午後12時(12 p.m.)Japanese common usageja:質問4-1）正午は午前12時？それとも、午後12時？ Question 4-1) Is noon 12 a.m.? Or 12 p.m.? https://www.nao.ac.jp/faq/a0401.html National Astronomical Observatory of Japan ja 19 January 2022 ja 午前0時(0 a.m.)ja 午後0時(0 p.m.)ja 午後12時(12 p.m.)Canadian Press, UK standardmidnightnoonmidnightAssociated Press stylePaula 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, 978-1-5416-9989-2, s.v. noon, midnight, times.—noonmidnightNISTmidnightniststartend NIST recommends using 11:59 p.m. and 12:01 a.m. to disambiguate when needed.12:01 a.m.noonmidnightniststartend11:59 p.m. It is not always clear what times "12:00 a.m." and "12:00 p.m." denote. In Latin, la ante meridiem (a.m.) means "before midday" and 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. Although noon could be denoted "12 m.", this is seldom done Chicago Manual of Style paragraph 9.40 18th 2024 University of Chicago Press Although noon can be expressed as 12:00 m. (m = meridies), very few use that form. 978-0-226-81797-2 and also does not resolve the question of how to indicate midnight. By convention, "12 a.m." denotes midnight and "12 p.m." denotes noon.http://ahdictionary.com/word/search.html?q=AM AM https://web.archive.org/web/20140109230904/http://ahdictionary.com/word/search.html?q=AM 9 January 2014 live The American Heritage Dictionary of the English Language Fifth 2011 However, many style guides recommend against using either because of the potential for confusion. Many recommend instead using the unambiguous terms "12 noon" and "12 midnight", or simply "noon" and "midnight". These include The American Heritage Dictionary of the English Language, The Canadian Press Stylebook,The Canadian Press Stylebook 11th 1999 page 288 and the NIST's "Frequently asked questions (FAQ)" web page.https://www.nist.gov/pml/time-and-frequency-division/times-day-faqs Times of Day FAQs National Institute of Standards and Technology 21 September 2016 30 September 2018 https://web.archive.org/web/20180921153333/https://www.nist.gov/pml/time-and-frequency-division/times-day-faqs 21 September 2018 live Alternatively, some recommend referring to one minute before or after 12:00, especially when referring to midnight (for example, "11:59 p.m." or "12:01 a.m."). These include the UK's National Physical Laboratory "FAQ-Time" web page.http://www.npl.co.uk/science-technology/time-frequency/time/faqs National Physical Laboratory, FAQ-Time 11 January 2015 https://web.archive.org/web/20150303095129/http://www.npl.co.uk/science-technology/time-frequency/time/faqs/ 3 March 2015 dead That has become common in the United States in legal contracts and for airplane, bus, or 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 p.m. or 12:01 a.m.http://www.abelliogreateranglia.co.uk/f/62130/62130.pdf Cheshunt/Enfield Town to London Liverpool Street via Seven Sisters service Interim train timetables https://web.archive.org/web/20150526154117/http://www.abelliogreateranglia.co.uk/f/62130/62130.pdf 26 May 2015 dead Abellio Greater Anglia London 17 May 2015 7,8 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 a.m. means noon and 12 p.m. means midnight.Richards E. G. Mapping Time: the Calendar and its History Oxford University Press 1999 289 Historically, the style manual of the United States Government Printing Office used 12 a.m. for noon and 12 p.m. for midnight, though this was reversed in its 2008 editions. See also 30em 24-hour clock Canonical hours Clock position Date and time representation by country Decimal time Italian six-hour clock Midnight Muhurta Noon Pahar Thai six-hour clock Notes References External links NIST FAQ on time https://web.archive.org/web/20120916120545/http://www.nist.gov/pml/div688/times.cfm 16 September 2012 12am is noon in Japan Category:Date and time representation Category:Time measurement systems