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24-hour clock

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A public 24-hour clock inCuritiba,Brazil, with the hour hand on the outside and the minute hand on the inside.

The modern24-hour clockis the convention oftimekeepingin which thedayruns frommidnightto midnight and is divided into 24 hours. This is indicated by the hours (and minutes) passed since midnight, from00(:00)to23(:59),with24(:00)as an option to indicate the end of the day. This system, as opposed to the12-hour clock,is the most commonly used time notation in the world today,[A]and is used by the international standardISO 8601.[1]

A number of countries, particularly English speaking, use the 12-hour clock, or a mixture of the 24- and 12-hour time systems. In countries where the 12-hour clock is dominant, some professions prefer to use the 24-hour clock. For example, in the practice ofmedicine,the 24-hour clock is generally used in documentation of care as it prevents anyambiguityas to when events occurred in a patient'smedical history.[2]

Description

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24-hour clock 12-hour clock
00:00 12:00 midnight[a]
12:00 a.m. (start of the day)
01:00 1:00 a.m.
02:00 2:00 a.m.
03:00 3:00 a.m.
04:00 4:00 a.m.
05:00 5:00 a.m.
06:00 6:00 a.m.
07:00 7:00 a.m.
08:00 8:00 a.m.
09:00 9:00 a.m.
10:00 10:00 a.m.
11:00 11:00 a.m.
12:00 12:00 noon[a]
12:00 p.m.
13:00 1:00 p.m.
14:00 2:00 p.m.
15:00 3:00 p.m.
16:00 4:00 p.m.
17:00 5:00 p.m.
18:00 6:00 p.m.
19:00 7:00 p.m.
20:00 8:00 p.m.
21:00 9:00 p.m.
22:00 10:00 p.m.
23:00 11:00 p.m.
24:00 equals
00:00 next day		 12:00 midnight[a]
(end of the day)
  1. ^abcSee:12-hour clock § Confusion at noon and midnight
World map showing the usage of 12 or 24-hour clock in different countries
24-hour
24-hour (12-hour orally)
Both in common use
12-hour
24-hour digital clock inMiaoli HSR station.

A time of day is written in the 24-hour notation in the form hh:mm (for example 01:23) or hh:mm:ss (for example, 01:23:45), where hh (00 to 23) is the number of full hours that have passed sincemidnight,mm (00 to 59) is the number of full minutes that have passed since the last full hour, and ss (00 to 59) is the number of seconds since the last full minute. To indicate the exact end of the day, hh may take the value 24, with mm and ss taking the value 00. In the case of aleap second,the value of ss may extend to 60. A leading zero is added for numbers under 10, but it is optional for the hours. The leading zero is very commonly used in computer applications, and always used when a specification requires it (for example,ISO 8601).

Where subsecond resolution is required, the seconds can be adecimal fraction;that is, the fractional part follows a decimal dot or comma, as in 01:23:45.678. The most commonly used separator symbol between hours, minutes and seconds is thecolon,which is also the symbol used in ISO 8601. In the past, some European countries used thedot on the lineas a separator, but most national standards on time notation have since then been changed to the international standard colon. In some contexts (including some computer protocols and military time), no separator is used and times are written as, for example, "2359".

Midnight 00:00 and 24:00

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"23:59" redirects here. For the Christian metal album, seeVeni Domine § Discography.For the film, see23:59 (film).

In the 24-hour time notation, the day begins at midnight, 00:00 or 0:00, and the last minute of the day begins at 23:59. Where convenient, the notation 24:00 may also be used to refer to midnight at the end of a given date[3]— that is, 24:00 of one day is the same time as 00:00 of the following day.

The notation 24:00 mainly serves to refer to the exact end of a day in a time interval. A typical usage is giving opening hours ending at midnight (e.g. "00:00–24:00", "07:00–24:00" ). Similarly, some bus and train timetables show 00:00 as departure time and 24:00 as arrival time. Legal contracts often run from the start date at 00:00 until the end date at 24:00.

While the 24-hour notation unambiguously distinguishes between midnight at the start (00:00) and end (24:00) of any given date, there is no commonly accepted distinction among users of the 12-hour notation.Style guidesand military communication regulations in some English-speaking countries discourage the use of 24:00 even in the 24-hour notation, and recommend reporting times near midnight as 23:59 or 00:01 instead.[4]Sometimes the use of 00:00 is also avoided.[4]In variance with this, as of 2010, the correspondence manual for theUnited States NavyandUnited States Marine Corpsformerly specified 0001 to 2400.[5]The manual was updated in June 2015 to use 0000 to 2359.[6]

Times after 24:00

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See also:Date and time notation in Japan § Time

Time-of-day notations beyond 24:00 (such as 24:01 or 25:00 instead of 00:01 or 01:00) are not commonly used and not covered by the relevant standards. However, they have been used occasionally in some special contexts in the United Kingdom, France, Spain, Canada, Japan, South Korea, Hong Kong, and China where business hours extend beyond midnight, such as broadcast television production and scheduling. TheGTFSpublic transport schedule listings file format has the concept of service days and expects times beyond 24:00 for trips that run after midnight.[7]

Computer support

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In most countries, computers by default show the time in 24-hour notation. For example,Microsoft WindowsandmacOSactivate the 12-hour notation by default only if a computer is in a handful of specific language and region settings. The 24-hour system is commonly used in text-based interfaces.POSIXprograms such aslsdefault to displaying timestamps in 24-hour format.

Military time

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24-hour clock as seen on theUSS Midway.

InAmerican English,the termmilitary timeis a synonym for the 24-hour clock.[8]In the US, the time of day is customarily given almost exclusively using the 12-hour clock notation, which counts the hours of the day as 12, 1,..., 11 with suffixesa.m.andp.m.distinguishing the twodiurnalrepetitions of this sequence. The 24-hour clock is commonly used there only in some specialist areas (military, aviation, navigation, tourism, meteorology, astronomy, computing, logistics, emergency services, hospitals), where theambiguities of the 12-hour notationare deemed too inconvenient, cumbersome, or dangerous.

Military usage, as agreed between the United States and allied English-speaking military forces,[9]differs in some respects from other twenty-four-hour time systems:

  • No hours/minutes separator is used when writing the time, and aletter designating the time zoneis appended (for example "0340Z" ).
  • Leading zerosare always written out and are required to be spoken, so 5:43 a.m. is spoken "zero five forty-three" (casually) or "zero five four three" (military radio), as opposed to "five forty-three" or "five four three".
  • Military time zonesare lettered and given word designations from theNATO phonetic alphabet.For example, inUS Eastern Standard Time(UTC−5), which is designated time zone R, 2:00 a.m. is written "0200R" and spoken "zero two hundred Romeo".
  • Local time is designated as zone J or "Juliett"."1200J "(" twelve hundred Juliett ") is noon local time.
  • Greenwich Mean Time(GMT) orCoordinated Universal Time(UTC) is designated time zone Z, and thus called "Zulu time". (When used as a modern time zone, in practice, GMT and UTC coincide. For other purposes there may be a difference of about a second.[10])
  • Hours are always "hundred", never "thousand"; 1000 is "ten hundred" not "one thousand"; 2000 is "twenty hundred" not "two thousand".

History

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Paolo Uccello'sFace with Four Prophets/Evangelists(1443) inFlorence Cathedral
The 24-hour tower clock inVenicethat lists hours 1 to 12 twice

The first mechanical public clocks introduced in Italy weremechanical 24-hour clockswhich counted the 24 hours of the day from one-half hour after sunset to the evening of the following day. The 24th hour was the last hour of day time.[11]

From the 14th to the 17th century, two systems of time measurement competed in Europe:[12][13]

  • Italian (Bohemian, Old-Bohemian) hours (full-dial): 24 hours system with the day starting after sunset; on the static dial, the 24th hour was on the right side. In Italy, it was prevalently modified to a 4×6 hours system, but some 24-hour dials lasted until the 19th century. The system has spread especially to the Alpine countries, Czech countries and Poland. In Bohemia, this system was finally banned in 1621 after thedefeat on White Mountain.The Prague Astronomical Clock struck according to the Old Bohemian Clock until its destruction in 1945. The variant with counting from dawn is also rarely documented and used, e.g. on a 16th-century cabinet clock in the Vienna Art-History Museum.[14]
  • German (Gallic) hours (half-dial): 2×12 hour system starting at midnight and restarted at noon. It is typical with the 12-hour dial with 12 at the top.

The modern 24-hour system is a late-19th century adaptation of the German midnight-starting system, and then prevailed in the world with the exception of some Anglophone countries.

Striking clocks had to produce 300 strokes each day, which required a lot of rope, and wore out the mechanism quickly, so some localities switched to ringing sequences of 1 to 12 twice (156 strokes), or even 1 to 6 repeated four times (84 strokes).[11]

Sandford Fleming,the engineer-in-chief of the CanadianIntercolonial Railway,was an early proponent of using the 24-hour clock as part of a programme to reform timekeeping, which also included establishingtime zonesand a standardprime meridian.[15][16]At theInternational Meridian Conferencein 1884, the following resolution was adopted by the conference:[17]

That this universal day is to be a mean solar day; is to begin for all the world at the moment of midnight of the initial meridian coinciding with the beginning of the civil day and date of that meridian, and is to be counted from zero up to twenty-four hours.[17]

TheShepherd Gate ClockwithRoman numeralsup to XXIII (23) and 0 for midnight, in Greenwich

TheCanadian Pacific Railwaywas among the first organisations to adopt the 24-hour clock, atmidsummer1886.[15][18]A report by a government committee in the United Kingdom noted Italy as the first country among those mentioned to adopt 24-hour time nationally, in 1893.[19]Other European countries followed: France adopted it in 1912 (the French army in 1909), followed by Denmark (1916), and Greece (1917). By 1920, Spain, Portugal, Belgium, and Switzerland had switched, followed by Turkey (1925), and Germany (1927). By the early 1920s, many countries in Latin America had also adopted the 24-hour clock.[20]Some of the railways in India had switched before the outbreak of the war.[19]

DuringWorld War I,the British Royal Navy adopted the 24-hour clock in 1915, and the Allied armed forces followed soon after,[19]with the British Army switching officially in 1918.[21]The Canadian armed forces first started to use the 24-hour clock in late 1917.[22]In 1920, the United States Navy was the first United States organisation to adopt the system; theUnited States Army,however, did not officially adopt the 24-hour clock until 1 July 1942.[23] [24]

The use of the24-hour clock in the United Kingdomhas grown steadily since the beginning of the 20th century, although attempts to make the system official failed more than once.[25]In 1934, theBritish Broadcasting Corporation(BBC) switched to the 24-hour clock for broadcast announcements and programme listings. The experiment was halted after five months following a lack of enthusiasm from the public, and the BBC continued using the 12-hour clock.[25]In the same year,Pan American World Airways CorporationandWestern Airlinesin the United States both adopted the 24-hour clock.[26]In modern times, the BBC uses a mixture of both the 12-hour and the 24-hour clock.[25]British Rail,London Transport,and theLondon Undergroundswitched to the 24-hour clock for timetables in 1964.[25]A mixture of the 12- and 24-hour clocks similarly prevails in other English-speakingCommonwealthcountries: French speakers have adopted the24-hour clock in Canadamuch more broadly than English speakers, andAustraliaas well asNew Zealandalso use both systems.

See also

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Notes

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  1. ^See theCommon Locale Data Repositoryfor detailed data about the preferred date and time notations used across the world, as well thelocalesettings of major computeroperating systems,and the articleDate and time representation by country.

References

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  1. ^ISO 8601:2004 – Data elements and interchange formats – Information interchange – Representation of dates and times(3rd ed.). 2004.
  2. ^Pickar, Gloria D.; Graham, Hope; Swart, Beth; Swedish, Margaret (2011).Dosage calculations(2nd Canadian ed.). Toronto: Nelson Education. p. 60.ISBN9780176502591.
  3. ^ISO 8601:2004 – Data elements and interchange formats – Information interchange – Representation of dates and times(3rd ed.). 2004. Clause 4.2.3 "Midnight".
  4. ^abCommunication instructions – General(PDF).Combined Communications-Electronics Board. October 2010. p. 3-6. ACP 121(I). Archived fromthe original(PDF)on 7 August 2011.
  5. ^Department of the Navy Correspondence Manual(PDF).March 2010. Chapter 2, Section 5 Paragraph 15. Expressing Military Time. SECNAV M-5216.5 – viaUnited States Marines.
  6. ^Department of the Navy Correspondence Manual(PDF).June 2015. Chapter 2, Section 5 Paragraph 15. Expressing Military Time. SECNAV M-5216.5 – viaUnited States Marines.
  7. ^"Reference | Static Transit".Google Developers.
  8. ^"military time".LexicoUK English Dictionary.Oxford University Press.Archived fromthe originalon 12 November 2020.
  9. ^"Communication Instructions General ACP 121(I)"(PDF).Archived fromthe original(PDF)on 8 May 2016.
  10. ^Guinot, Bernard (August 2011). "Solar time, legal time, time in use".Metrologia.48(4): 185.Bibcode:2011Metro..48S.181G.doi:10.1088/0026-1394/48/4/S08.S2CID121852011.
  11. ^abDohrn-Van Rossum, Gerhard (1996).History of the Hour. Clock and Modern Temporal Orders.The University of Chicago Press. p. 114.ISBN0226155110.
  12. ^Tomáš Voříšek:Hodiny německé a české na půl orloje a podle celého orloje,Otta z Losu, last update 2021.
  13. ^Stan. Marušák, Petr Král:Čtyřiadvacetník,Orloj.eu.
  14. ^Český čas s otazníkem,Orloj.eu.
  15. ^abFleming, Sandford (1886)."Time-reckoning for the twentieth century".Annual Report of the Board of Regents of the Smithsonian Institution(1): 345–366.Reprinted in 1889:Time-reckoning for the twentieth centuryat theInternet Archive.
  16. ^Creet, Mario (1990)."Sandford Fleming and Universal Time".Scientia Canadensis: Canadian Journal of the History of Science, Technology and Medicine.14(1–2): 66–89.doi:10.7202/800302ar.
  17. ^ab"International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October 1884. Protocols of the proceedings".Project Gutenberg. 1884.Retrieved30 November2012.
  18. ^The LondonTimesreports on a timetable using the 24-hour clock on a trip fromPort Arthur, Ontario:"A Canadian Tour".The Times.No. 31880. London. 2 October 1886. col 1–2, p. 8.
  19. ^abc"Memorandum CAB 24/110/21 (CP 1721), 'Report of the Committee upon the 24 hour method of expressing time'".The National Archives,Kew,United Kingdom.4 August 1920.
  20. ^"History of the 24-Hour Clock - 24 Hour Time Clock".Military Time Chart.24 December 2016.Retrieved27 July2023.
  21. ^The Times:1918 September 19, p. 3.
  22. ^Dancocks, Daniel G.Gallant Canadians: The Story of the 10th Canadian Infantry Battalion 1914–1919.
  23. ^"Army Adopts 24-Hour Clock Plan".Brownsville Herald.Brownsville, Texas.26 June 1942. p. 2.Effective July 1, the United States Army will adopt the 24-hour clock system for all messages, dispatches, orders and reports, the War Department has announced. Under this new system time will be expressed in a group of four digits ranging from 0000 to 2400. [...] Flying officers atMoore Fieldhave used the 24-hour clock system for some time. However, its use in official business messages will be an innovation.
  24. ^"It Will Be 2100 O'Clock at Ft. Bliss At 3 p. m."El Paso Herald-Post.El Paso.3 July 1942. p. 3.The Army has a new time system — Greenwich Civil Time — to be used in radiograms, telegrams, and all official communications. [...] Effective 10 p. m. Monday, the War Department adopted the 24-hour system, which is used internationally with the Navy and the Armed forces of associated nations. [...] the six hours difference between Mountain War Time and actual Greenwich time must be taken into account, so that 12 noon is expressed on communications as 1800.
  25. ^abcdBoardman, Peter (July 2011).Counting Time: a brief history of the 24-hour clock.[self-published source]
  26. ^Sarasota Herald-Tribune 1943 May 14.
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