View source for Physics:24-hour clock

{{Short description|Timekeeping convention}}

[[File:24 hour analog clock rua 24 horas curitiba brasil.jpg|thumb|A public 24 hour clock in Curitiba, Brazil with the hour hand displayed on the outside and the minute hand shown on the inside.]]
The modern '''24-hour clock''' is the convention of timekeeping in which the [[Day|day]] runs from midnight to midnight and is divided into 24 hours. This is indicated by the hours (and minutes) passed since midnight, from 00(:00) to 23(:59). This system, as opposed to the [[Physics:12-hour clock|12-hour clock]], is the most commonly used time notation in the world today,{{Efn-ua|name=CLDR|See the [[Common Locale Data Repository]] for detailed data about the preferred date and time notations used across the world, as well the [[Software:Locale (computer software)|locale]] settings of major computer [[Operating system|operating system]]s, and the article Date and time representation by country.}} and is used by the international standard [[ISO 8601]].<ref name="iso8601">{{cite ISO standard |title=ISO 8601:2004 – Data elements and interchange formats – Information interchange – Representation of dates and times |csnumber=40874 |edition=3rd |date=2004}}</ref>

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 of [[Medicine:Medicine|medicine]], the 24-hour clock is generally used in documentation of care as it prevents any [[Ambiguity|ambiguity]] as to when events occurred in a patient's [[Medicine:Medical history|medical history]].<ref>{{Cite book |title=Dosage calculations |last1=Pickar |first1=Gloria D. |last2=Graham |first2=Hope |last3=Swart |first3=Beth |last4=Swedish |first4=Margaret |date=2011 |publisher=Nelson Education |isbn=9780176502591 |edition=2nd Canadian |location=Toronto |page=60}}</ref>

== Description ==
{| class="wikitable" style="float:right; margin:0.5em 0 0.5em 1em; font-size:95%;"
!24-hour clock||[[Physics:12-hour clock|12-hour clock]]
|-
| 00:00||12:00&nbsp;(midnight){{Efn|name=noonmidnight}}<br />12:00&nbsp;a.m. (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||12:00&nbsp;noon{{Efn|See: {{section link|12-hour clock|Confusion at noon and midnight}}|name=noonmidnight}}<br />12:00&nbsp;p.m.
|-
| 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 equals<br />00:00 next day||12:00 (midnight){{Efn|name=noonmidnight}}<br />(end of the day)
|-
| colspan="3"|{{notelist}}
|}
[[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}}]]
[[File:Digital clock in THSR Miaoli Station.jpg|thumb|24-hour digital clock in Miaoli 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 since midnight, 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. In the case of a leap 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 a [[Decimal#Decimal fractions|decimal 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 the colon, which is also the symbol used in ISO 8601. In the past, some European countries used the dot on the line as 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), no separator is used and times are written as, for example, "2359".

=== Midnight 00:00 and 24:00 ===
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<ref>{{cite ISO standard |title=ISO 8601:2004 – Data elements and interchange formats – Information interchange – Representation of dates and times |csnumber=40874 |edition=3rd |at=Clause 4.2.3 "Midnight" |date=2004}}</ref>&nbsp;— 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 guide]]s and 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.<ref name=acp121>{{cite book |url=http://jcs.dtic.mil/j6/cceb/acps/acp121/ACP121I.pdf |title=Communication instructions – General |id=ACP 121(I) |page=3{{hyp}}6 |publisher=Combined Communications-Electronics Board |date=October 2010 |archive-url=https://web.archive.org/web/20110807210103/http://jcs.dtic.mil/j6/cceb/acps/acp121/ACP121I.pdf |archive-date=7 August 2011}}</ref> Sometimes the use of 00:00 is also avoided.<ref name=acp121 /> In variance with this, as of 2010, the correspondence manual for the United States Navy and United States Marine Corps formerly specified 0001 to 2400.<ref>{{cite book |url=http://www.marforres.marines.mil/Portals/116/Docs/G-1/AAU/AAUDocuments/CORRESPONDENCE%20MANUAL.pdf |id=SECNAV M-5216.5 |title=Department of the Navy Correspondence Manual |date=March 2010 |website=United States Marines |at=Chapter 2, Section 5 Paragraph 15. Expressing Military Time}}</ref> The manual was updated in June 2015 to use 0000 to 2359.<ref>{{Cite book |url=https://www.marines.mil/Portals/59/Publications/SECNAV%20M%205216.5.pdf |id=SECNAV M-5216.5 |title=Department of the Navy Correspondence Manual |date=June 2015 |website=United States Marines |at=Chapter 2, Section 5 Paragraph 15. Expressing Military Time}}</ref>

=== Times after 24:00 ===
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. The [[General Transit Feed Specification|GTFS]] public transport schedule listings file format has the concept of service days and expects times beyond 24:00 for trips that run after midnight.<ref>{{Cite web|url=https://developers.google.com/transit/gtfs/reference|title=Reference &#124; Static Transit|website=Google Developers}}</ref>

=== Computer support ===
In most countries, computers by default show the time in 24-hour notation. For example, [[Software:Microsoft Windows|Microsoft Windows]] and [[Software:MacOS|macOS]] activate 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. [[POSIX]] programs such as [[Software:Ls|ls]] default to displaying timestamps in 24-hour format.

=== Military time ===
[[File:24-Hour Clock Aboard the USS Midway (8727193240).jpg|thumb|24-hour clock as seen on the USS Midway.]]
In American English, the term ''military time'' is a synonym for the 24-hour clock.<ref>{{Cite encyclopedia |url=http://www.lexico.com/definition/military_time |archive-url=https://web.archive.org/web/20201112033709/https://www.lexico.com/definition/military_time |url-status=dead |archive-date=12 November 2020 |title=military time |dictionary=Lexico UK English Dictionary |publisher=[[Organization:Oxford University Press|Oxford University Press]]}}</ref> 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 suffixes ''a.m.'' and ''p.m.'' distinguishing the two [[Day|diurnal]] repetitions 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 the [[Physics:12-hour clock#Confusion at noon and midnight|ambiguities of the 12-hour notation]] are deemed too inconvenient, cumbersome, or dangerous.

Military usage, as agreed between the United States and allied English-speaking military forces,<ref>{{Cite web
| title=Communication Instructions General ACP 121(I)
| url=http://jcs.dtic.mil/j6/cceb/acps/acp121/ACP121I.pdf
| url-status=dead
| archive-url=https://web.archive.org/web/20160508202743/http://jcs.dtic.mil/j6/cceb/acps/acp121/ACP121I.pdf
| archive-date=8 May 2016
}}</ref> differs in some respects from other twenty-four-hour time systems:
*No hours/minutes separator is used when writing the time, and a [[Nautical time#Letter suffixes|letter designating the time zone]] is appended (for example "0340Z").
*[[Leading zero]]s are always written out and are required to be spoken, so 5:43&nbsp;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".
*[[List of military time zones|Military time zones]] are lettered and given word designations from the [[Social:NATO phonetic alphabet|NATO phonetic alphabet]]. For example, in [[Eastern Time Zone|US Eastern Standard Time]] (UTC−5), which is designated time zone R, 2:00&nbsp;a.m. is written "0200R" and spoken "zero two hundred Romeo".
*Local time is designated as zone J or "[[Social:NATO phonetic alphabet|Juliett]]". "1200J" ("twelve hundred Juliett") is noon local time.
*[[Greenwich Mean Time]] (GMT) or [[Physics:Coordinated Universal Time|Coordinated 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.<ref>{{Cite journal| last1 = Guinot| first1 = Bernard | date = August 2011 | volume=48 | issue=4 | title = Solar time, legal time, time in use | journal = Metrologia | page =185 | doi = 10.1088/0026-1394/48/4/S08|bibcode = 2011Metro..48S.181G| s2cid = 121852011 }}</ref>)
*Hours are always "hundred", never "thousand"; 1000 is "ten hundred" not "one thousand"; 2000 is "twenty hundred" not "two thousand".

== History ==
[[File:Florence-Duomo-Clock.jpg|thumb|Paolo Uccello's ''Face with Four Prophets/Evangelists'' (1443) in the Florence Cathedral]]
[[File:Clock 24 J.jpg|thumb|The 24-hour tower clock in Venice that lists hours 1 to 12 twice]]
The first mechanical public clocks introduced in Italy were mechanical 24-hour clocks which 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.<ref name="Rossum">{{Cite book |last=Dohrn-Van Rossum |first=Gerhard |date=1996 |title=History of the Hour. Clock and Modern Temporal Orders |publisher=The University of Chicago Press |page=114 |isbn=0226155110}}</ref>

From the 14th to the 17th century, two systems of time measurement competed in Europe:<ref>Tomáš Voříšek: [http://otta.cechove.cz/hodiny.htm Hodiny německé a české na půl orloje a podle celého orloje], Otta z Losu, last update 2021.</ref><ref>Stan. Marušák, Petr Král: [https://www.orloj.eu/cs/orloj_ctyriadvacetnik.htm Čtyřiadvacetník], Orloj.eu.</ref>
* Italian (Bohemian, Old-Bohemian) hours (full-dial): 24 hours system with the day starting after sunset; on the static dial, the 24th hour was situated 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 the defeat 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.<ref>[https://www.orloj.eu/cs/cesky_cas.htm Český čas s otazníkem], Orloj.eu.</ref>
* 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).<ref name="Rossum" />

Sandford Fleming, the engineer-in-chief of the Canadian Intercolonial Railway, was an early proponent of using the 24-hour clock as part of a programme to reform timekeeping, which also included establishing [[Time zone|time zone]]s and a standard [[Earth:Prime meridian|prime meridian]].<ref name="Fleming">{{Cite journal| issue = 1| pages = 345–366| last = Fleming| first = Sandford| title = Time-reckoning for the twentieth century| journal = Annual Report of the Board of Regents of the Smithsonian Institution| date = 1886| url = https://library.si.edu/digital-library/book/annualreportofbo18861smit}} Reprinted in 1889: {{Internet Archive|timereckoningfor00flem|''Time-reckoning for the twentieth century''}}.</ref><ref name="Creet">{{Cite journal |last1=Creet |first1=Mario |title=Sandford Fleming and Universal Time |journal=Scientia Canadensis: Canadian Journal of the History of Science, Technology and Medicine |date=1990 |volume=14 |issue=1–2 |pages=66–89 |doi=10.7202/800302ar|doi-access=free }}</ref> At the International Meridian Conference in 1884, the following resolution was adopted by the conference:<ref name=imc />
{{blockquote|
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.<ref name=imc>{{Cite web|title=International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day. October 1884. Protocols of the proceedings.|url=http://www.gutenberg.org/files/17759/17759-h/17759-h.htm|publisher=Project Gutenberg|access-date=30 November 2012|year=1884}}</ref>
}}

[[File:Greenwich clock 1-manipulated.jpg|thumb|The Shepherd Gate Clock with [[Roman numerals|Roman numerals]] up to XXIII (23) and 0 for midnight, in Greenwich]]
The [[Company:Canadian Pacific Railway|Canadian Pacific Railway]] was among the first organisations to adopt the 24-hour clock, at midsummer 1886.<ref name="Fleming" /><ref>The London ''Times'' reports on a timetable using the 24-hour clock on a trip from Port Arthur, Ontario: {{Cite newspaper The Times |title= A Canadian Tour |date= 2 October 1886 |page=8 |issue=31880 |column=1–2}}</ref> 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.<ref name="baird">{{Cite web|url=http://discovery.nationalarchives.gov.uk/details/r/D7736214|title=Memorandum CAB 24/110/21 (CP 1721), 'Report of the Committee upon the 24 hour method of expressing time'|date=4 August 1920|location=The National Archives, Kew, {{wipe|United Kingdom}}}}</ref> 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.<ref>{{Cite web|url=https://militarytimechart.net/history-24-hour-clock/|website=Military Time Chart|title=History of the 24-Hour Clock - 24 Hour Time Clock|date=24 December 2016|accessdate=27 July 2023}}</ref> Some of the railways in India had switched before the outbreak of the war.<ref name="baird" />

During [[Social:World War I|World War I]], the British Royal Navy adopted the 24-hour clock in 1915, and the Allied armed forces followed soon after,<ref name="baird" /> with the British Army switching officially in 1918.<ref>''The Times'': 1918 September 19, p. 3.</ref> The Canadian armed forces first started to use the 24-hour clock in late 1917.<ref>Dancocks, Daniel G. ''Gallant Canadians: The Story of the 10th Canadian Infantry Battalion 1914–1919''.</ref> In 1920, the United States Navy was the first United States organisation to adopt the system; the United States Army, however, did not officially adopt the 24-hour clock until 1 July 1942.<ref name="BrownsvilleHerald1942">{{cite news |title=Army Adopts 24-Hour Clock Plan |newspaper=Brownsville Herald |location=Brownsville, Texas |date=26 June 1942 |page=2 |url=https://newspaperarchive.com/us/texas/brownsville/brownsville-herald/1942/06-26/page-2/ |quote=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 at Moore Field have used the 24-hour clock system for some time. However, its use in official business messages will be an innovation.}}</ref>
<ref name="ElPaso1942">{{cite news |title=It Will Be 2100 O'Clock at Ft. Bliss At 3 p. m. |newspaper=El Paso Herald-Post |location=El Paso |date=3 July 1942 |page=3 |url=https://newspaperarchive.com/us/texas/el-paso/el-paso-herald-post/1942/07-03/page-3/ |quote=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.}}</ref>

The use of the 24-hour clock in the United Kingdom has grown steadily since the beginning of the 20th century, although attempts to make the system official failed more than once.<ref name="counting-time">{{Cite book |last=Boardman |first=Peter |date=July 2011 |url=http://clock24.nfshost.com/counting-time.html |title=Counting Time: a brief history of the 24-hour clock}}{{Self-published source|date=September 2014}}</ref> In 1934, the British 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.<ref name="counting-time" /> In the same year, Pan American World Airways Corporation and Western Airlines in the United States both adopted the 24-hour clock.<ref>Sarasota Herald-Tribune 1943 May 14.</ref> In modern times, the BBC uses a mixture of both the 12-hour and the 24-hour clock.<ref name="counting-time" /> British Rail, London Transport, and the London Underground switched to the 24-hour clock for timetables in 1964.<ref name="counting-time" /> A mixture of the 12- and 24-hour clocks similarly prevails in other English-speaking [[Organization:Commonwealth of Nations|Commonwealth]] countries: French speakers have adopted the 24-hour clock in Canada much more broadly than English speakers, and Australia as well as New Zealand also use both systems.

== See also ==
{{div col|colwidth=23em}}
* [[Physics:12-hour clock|12-hour clock]]
* 24-hour analog dial
* [[Physics:Clock|Clock]]
* Date and time representation by country
* [[Decimal time]]
* [[Hindu units of time]]
* [[Physics:Italian six-hour clock|Italian six-hour clock]]
* [[List of military time zones]]
* [[Metric time]]
* [[Physics:Thai six-hour clock|Thai six-hour clock]]
* [[Time]]
* [[Astronomy:Traditional Chinese timekeeping|Traditional Chinese timekeeping]]
{{div col end}}

== Notes ==
{{notelist-ua}}

== References ==
{{Reflist}}

== External links ==
* [http://clock24.nfshost.com/counting-time.html ''Counting Time: a brief history of the 24-hour clock'' (free e-book)]
{{Time measurement and standards}}

{{DEFAULTSORT:24-Hour Clock}}
[[Category:Time measurement systems]]

{{Sourceattribution|24-hour clock}}