Editing Universal Time (section)

==History==
Prior to the introduction of [[standard time]], each municipality throughout the clock-using world set its official clock, if it had one, according to the local position of the Sun (see [[solar time]]). This served adequately until the introduction of [[rail travel in Britain]], which made it possible to travel fast enough over sufficiently long distances as to require continuous re-setting of [[clock|timepiece]]s as a [[train]] progressed in its daily run through several towns. Starting in 1847, Britain established [[Greenwich Mean Time]], the [[mean solar time]] at [[Greenwich, England]], to solve this problem: all clocks in Great Britain were set to this time regardless of local solar noon.{{efn|Despite its mandatory use on [[Great Western Railway]] stations from 1847 and thus widespread informal adoption, it was not until the [[Statutes (Definition of Time) Act 1880]] that it became law.<ref name=londontime>{{cite web|url=https://londonist.com/london/history/why-britain-sets-its-clocks-to-london | title = Why Britain Sets Its Clocks To London | author = Harry Rosehill | website = Londonist.com | date = 31 May 2017 | access-date= 25 November 2019}}</ref> The act declared that standard time in Great Britain was to be Greenwich [[Solar time#Mean time|Mean Time]] and standard time in Ireland was to be Dublin time.}} Using telescopes, GMT was calibrated to the [[mean solar time]] at [[prime meridian (Greenwich)|the prime meridian]] through the [[Royal Observatory, Greenwich]]. [[Marine chronometer|Chronometer]]s or [[telegraphy]] were used to synchronize these clocks.{{sfn|Howse|1997|loc=ch. 4}}

[[File:World_Time_Zones_Map.png|thumb|center|upright 2.5|Standard time zones of the world. The number at the bottom of each zone specifies the number of hours to add to UTC to convert it to the local time.]]
As international commerce increased, the need for an international standard of time measurement emerged. Several authors proposed a "universal" or "cosmic" time (see {{section link|Time zone#Worldwide time zones}}). The development of Universal Time began at the [[International Meridian Conference]]. At the end of this conference, on 22 October 1884,{{efn|Voting took place on 13 October.}} the recommended base reference for world time, the "universal day", was announced to be the local mean solar time at the Royal Observatory in Greenwich, counted from 0 hours at Greenwich mean midnight.{{sfn|Howse|1997|pp=12, 137}} This agreed with the civil Greenwich Mean Time used on the island of Great Britain since 1847. In contrast, astronomical GMT began at mean noon, i.e. astronomical day ''X'' began at noon of civil day ''X''. The purpose of this was to keep one night's observations under one date. The civil system was adopted as of 0 hours (civil) 1 January 1925. Nautical GMT began 24 hours before astronomical GMT, at least until 1805 in the [[Royal Navy]], but persisted much later elsewhere because it was mentioned at the 1884 conference. Greenwich was chosen because by 1884 two-thirds of all [[nautical chart]]s and [[map]]s already used it as their [[prime meridian]].{{sfn|Howse|1997|p=[https://archive.org/details/greenwichtimelon0000hows/page/133 133–137]}}

During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.{{sfn|Howse|1997|loc=ch. 6}}

In 1928, the term ''Universal Time'' (''UT'') was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight.{{sfn|McCarthy|Seidelmann|2009|pp=10–11}} The term was recommended as a more precise term than ''Greenwich Mean Time'', because ''GMT'' could refer to either an [[astronomical day]] starting at noon or a civil day starting at midnight.{{sfn|McCarthy|Seidelmann|2009|p=14}} As the general public had always begun the day at midnight, the timescale continued to be presented to them as Greenwich Mean Time.{{citation needed|date=February 2024}}

When introduced, broadcast [[time signal]]s were based on UT, and hence on the rotation of the Earth. In 1955 the [[International Time Bureau|BIH]] adopted a proposal by William Markowitz, effective 1 January 1956, dividing UT into UT0 (UT as formerly computed), UT1 (UT0 corrected for polar motion) and UT2 (UT0 corrected for polar motion and seasonal variation). UT1 was the version sufficient for "many astronomical and geodetic applications", while UT2 was to be broadcast over radio to the public.<ref>{{cite web |last1=Dick |first1=Steven |last2=McCarthy |first2=Dennis | author-link2=Dennis McCarthy (scientist) |title=William Markowitz Obituary |url=https://ad.usno.navy.mil/wds/history/markowitz.html |website=U.S. Naval Observatory |date=15 December 2018|archive-url=https://web.archive.org/web/20181215224810/https://ad.usno.navy.mil/wds/history/markowitz.html |archive-date=15 December 2018 }}</ref><ref>{{cite web |title=On the Determination of Universal Time by the Time Services according to the Decisions of the General Assembly of the I.A.U. in Dublin |url=https://www.ucolick.org/~sla/leapsecs/BHs4n4EN.pdf |publisher=Bulletin Horaire |date=July–August 1955|archive-url=https://web.archive.org/web/20211215230325/https://www.ucolick.org/~sla/leapsecs/BHs4n4EN.pdf |archive-date=15 December 2021 |url-status=live}}</ref>

UT0 and UT2 soon became irrelevant due to the introduction of [[Coordinated Universal Time]] (UTC). Starting in 1956, [[WWV (radio station)|WWV]] broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1.{{sfn|Arias|Guinot|Quinn|2003}} The up to 20 ms error from UT1 is on the same order of magnitude as the differences between UT0, UT1, and UT2. By 1960, the U.S. Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory had developed UTC, with a similar stepping approach. The 1960 URSI meeting recommended that all time services should follow the lead of the UK and US and broadcast coordinated time using a frequency offset from cesium aimed to match the predicted progression of UT2 with occasional steps as needed.<ref name="sla">{{cite web |title=Seasonal Variation of Earth Rotation |url=https://www.ucolick.org/~sla/leapsecs/seasonal.html|first=Steve|last=Allen|website=ucolick.org}}</ref> Starting 1 January 1972, UTC was defined to follow UT1 within 0.9 seconds rather than UT2, marking the decline of UT2.{{sfn|McCarthy|Seidelmann|2009|loc=Ch. 14}}

Modern [[civil time]] generally follows UTC. In some countries, the term ''Greenwich Mean Time'' persists in common usage to this day in reference to UT1, in [[civil time]]keeping as well as in astronomical almanacs and other references. Whenever a level of [[accuracy]] better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC is known as [[DUT1]].{{sfn|McCarthy|Seidelmann|2009|loc=Ch. 14}}

===Adoption in various countries===
The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.
{{Col-begin}}
{{Col-break}}
{|
! width=60 align=left | Year 
!  align=left | Countries<ref>{{harvnb|Howse|1980|pp=154–5}}. Names have not been updated.</ref>
|-
| 1847 || Great Britain<ref name=londontime/>
|-
| 1880 || Ireland (entire island)
|-
| 1883 || Canada, United States{{efn|legal in 1918 ([[Standard Time Act]])}}
|-
| 1884 || Serbia 
|-
| 1886 || New Zealand<ref>{{Cite news |last=Stafford |first=E. W. |date=30 October 1868 |title=The New Zealand Gazette |url=https://library.victoria.ac.nz/databases/nzgazettearchive/pubs/gazettes/1868/1868%20ISSUE%20061.pdf }}</ref>
|-
| 1888 || Japan 
|-
| 1892 || Belgium, the Netherlands,{{efn|Legal time reverted to Amsterdam time 1909; to Central European Time 1940.}} S. Africa{{efn|except Natal}} 
|-
| 1893 || Italy, Germany, Austria-Hungary (railways) 
|-
| 1894 || Bulgaria, Denmark, Norway, Switzerland, Romania, Turkey (railways) 
|-
| 1895 || Australia, Natal 
|-
| 1896 || Formosa (Taiwan)
|-
| 1899 || Puerto Rico, Philippines 
|-
| 1900 || Sweden, Egypt, Alaska 
|-
| 1901 || Spain 
|-
| 1902 || Mozambique, Rhodesia 
|-
| 1903 || Ts'intao, Tientsin 
|-
| 1904 || China Coast, Korea, Manchuria, N. Borneo 
|-
| 1905 || Chile 
|-
| 1906 || India (except Calcutta), Ceylon (Sri Lanka), Seychelles 
|-
| 1907 || Mauritius, Chagos 
|-
| 1908 || Faroe Is., Iceland 
|-
| 1911 || France, Algeria, Tunis, many French overseas possessions, British West Indies 
|-
| 1912 || Portugal and overseas possessions, other French possessions, Samoa, Hawaii, Midway and Guam, Timor, Bismarck Arch., Jamaica, Bahamas Is. 
|-
| 1913 || British Honduras, Dahomey 
|-
| 1914 || Albania, Brazil, Colombia 
|-
| 1916 || Greece, Poland, Turkey 
|}
{{Col-break}}
{|
! width=60 align=left | Year 
!  align=left | Countries
|-
| 1917 || Iraq, Palestine 
|-
| 1918 || Guatemala, Panama, Gambia, Gold Coast 
|-
| 1919 || Latvia, Nigeria 
|-
| 1920 || Argentina, Uruguay, Burma, Siam 
|-
| 1921 || Finland, Estonia, Costa Rica 
|-
| 1922 || Mexico 
|-
| 1924 || Java, USSR 
|-
| 1925 || Cuba 
|-
| 1928 || China Inland 
|-
| 1930 || Bermuda 
|-
| 1931 || Paraguay 
|-
| 1932 || Barbados, Bolivia, Dutch East Indies 
|-
| 1934 || Nicaragua, E. Niger 
|-
| By 1936 || Labrador, Norfolk I. 
|-
| By 1937 || Cayman Is., Curaçao, Ecuador, Newfoundland 
|-
| By 1939 || Fernando Po, Persia 
|-
| By 1940 || Lord Howe I. 
|-
| 1940 || The Netherlands 
|-
| By 1948 || Aden, Ascension I., Bahrain, British Somaliland, Calcutta, Dutch Guiana, Kenya, Federated Malay States, Oman, Straits Settlements, St. Helena, Uganda, Zanzibar 
|-
| By 1953 || Rarotonga, South Georgia 
|-
| By 1954 || Cook Is. 
|-
| By 1959 || Maldive I. Republic 
|-
| By 1961 || Friendly Is., Tonga Is. 
|-
| By 1962 || Saudi Arabia 
|-
| By 1964 || Niue Is. 
|-
| 1972 || Liberia 
|}
{{col-end}}

Apart from [[Nepal Standard Time]] (UTC+05:45), the [[Chatham Standard Time Zone]] (UTC+12:45) used in New Zealand's [[Chatham Islands]]{{sfn|HM Nautical Almanac Office|2015}} and the officially unsanctioned [[Eucla, Western Australia#Time zone|Central Western Time Zone]] (UTC+8:45) used in [[Eucla, Western Australia]] and surrounding areas, all time zones in use are defined by an offset from UTC that is a multiple of half an hour, and in most cases a multiple of an hour.{{citation needed|date=February 2024}}