Editing International Date Line (section)

==Geography==
[[File:Dateline-animation-3deg-borderonly-180px.gif|thumb|upright=1.3|A simplified illustration of the relation between the International Date Line, the date, and the time of day. Each color represents a different date.]]
[[File:international date line.png|thumb|280px|Example depicting situation at 04:00 [[Greenwich Mean Time|GMT]] Tuesday. (Times are approximate, since time zone boundaries generally do not exactly coincide with meridians. Night and day is illustrative only; daylight hours depend on latitude and time of year.)]]

===Circumnavigating the globe===

People traveling westward [[circumnavigation|around the world]] must set their clocks: 
*Back by one hour for every 15° of longitude crossed, and 
*Forward by 24 hours upon crossing the International Date Line.

People traveling eastward must set their clocks: 
*Forward by one hour for every 15° of longitude crossed, and 
*Back by 24 hours upon crossing the International Date Line.

Moving forward or back 24 hours generally also implies a one day date change.

The 14th century Arab geographer [[Abulfeda]] predicted that circumnavigators would accumulate a one-day offset to the local date.<ref>{{cite book |last1=Gunn |first1=Geoffrey C. |title=Overcoming Ptolemy: The Revelation of an Asian World Region |date=15 October 2018 |publisher=Lexington Books |location=Lanham, Maryland |isbn=9781498590143 |pages=47–48}}</ref> This phenomenon was confirmed in 1522 at the end of the [[Magellan's circumnavigation|Magellan–Elcano expedition]], the first successful circumnavigation. After sailing westward around the world from [[Spain]], the expedition called at [[Cape Verde]] for provisions on Wednesday, 9 July 1522 (ship's time). However, the locals told them that it was actually Thursday, 10 July 1522. The crew was surprised, as they had recorded each day of the three-year journey without omission.<ref>{{cite book|last=Neal|first=Larry|title=The Rise of Financial Capitalism: International Capital Markets in the Age of Reason|url=https://books.google.com/books?id=ey8h1WC5w7kC|year=1993|publisher=Cambridge University Press|isbn=978-0-521-45738-5|page=[https://books.google.com/books?id=ey8h1WC5w7kC&dq=%229+july%22+%2210+july%22&pg=PA8-IA1 1]}}</ref> Cardinal [[Gasparo Contarini]], the Venetian ambassador to Spain, was the first European to give a correct explanation of the discrepancy.<ref>{{cite book |last1=Winfree |first1=Arthur T. |title=The Geometry of Biological Time |edition=2nd |date=2001 |publisher=Springer Science & Business Media |location=New York |isbn=978-1-4757-3484-3 |page=10 |language=en}}</ref>

===Description===
{{refimprove section|date=July 2022}}
:''This description is based on the most common understanding of the ''de facto'' International Date Line. See {{slink||De facto and de jure date lines}} below, and map above at right.''
The IDL is roughly based on the [[Meridian (geography)|meridian]] of 180° longitude, roughly down the middle of the Pacific Ocean, and halfway around the world from the [[IERS Reference Meridian]], the successor to the historic [[Prime meridian (Greenwich)|Greenwich prime meridian]] running through the [[Royal Observatory, Greenwich|Royal Greenwich Observatory]]. In many places, the IDL follows the 180° meridian exactly. In other places, however, the IDL deviates east or west away from that meridian. These various deviations generally accommodate the political and economic affiliations of the affected areas.

Proceeding from north to south, the first deviation of the IDL from 180° is to pass to the east of [[Wrangel Island]] and the [[Chukchi Peninsula]], the easternmost part of Russian [[Siberia]]. (Wrangel Island lies directly on the meridian at 71°32′N 180°0′E, also noted as 71°32′N 180°0′W.)<ref name="wrengell">{{cite journal |title=Arctic Expeditions Commanded by Americans |journal=The National Geographic Magazine |volume=18 |pages=459–468 |url=https://books.google.com/books?id=_RAOAQAAIAAJ&pg=PA459|year=1907 |access-date=4 January 2017}}</ref> It then passes through the [[Bering Strait]] between the [[Diomede Islands]] at a distance of {{convert|1.5|km|mi}} from each island at 168°58′37″ W.<ref name="Russia-US maritime boundary">{{citation |last=Allen |first=Jared |date=11 January 2012 |title=United States – Russian Maritime Boundary and Exclusive Economic Zones |website=ArcticEcon |url=https://arcticecon.wordpress.com/2012/01/11/united-states-russian-maritime-boundary-and-exclusive-economic-zones/ |access-date=4 January 2017 |archive-date=5 January 2017 |archive-url=https://web.archive.org/web/20170105181513/https://arcticecon.wordpress.com/2012/01/11/united-states-russian-maritime-boundary-and-exclusive-economic-zones/ |url-status=live }}</ref> It then bends considerably west of 180°, passing west of [[St. Lawrence Island]] and [[St. Matthew Island]].

The IDL crosses between the U.S. [[Aleutian Islands]] ([[Attu Island]] being the westernmost) and the [[Commander Islands]], which belong to Russia. It then bends southeast again to return to 180°. Thus, all of Russia is to the west of the IDL, and all of the United States is to the east except for the insular areas of [[Guam]], the [[Northern Mariana Islands]], and [[Wake Island]], reaching the hypothetical, but not used UTC–13:00 time zone.

The IDL remains on the 180° meridian until passing the [[equator]]. Two U.S.-owned uninhabited atolls, [[Howland Island]] and [[Baker Island]], just north of the equator in the central Pacific Ocean (and ships at sea between 172.5°W and 180°), have the ''earliest''{{efn|'''Earliest time on Earth''' here means that calendar day and clock time in this [[time zone]] is ''least'' advanced (or the {{nowrap|"{{em|oldest}}{{tsp}}")}} on Earth and that this place is the ''last'' {{nowrap|(''latest''{{tsp}})}} on Earth to reach any day and time. Because of that this time zone ([[UTC−12:00|IDLW]]) is sometimes also called [[Anywhere on Earth]] and deadlines can be set based on it e.g. "until the end of day Anywhere on Earth" means that if the day ended there it also ended everywhere else on the planet before that.<ref>{{Cite web|url=http://www.ieee802.org/16/aoe.html|title=IEEE 802.16 AOE Deadline Documentation|website=ieee802.org}}</ref>}} time on Earth ([[UTC−12:00]] hours).{{r|timeNearIDL1}}

The IDL circumscribes [[Kiribati]] by swinging far to the east, almost reaching the [[150th meridian west|150°W meridian]]. Kiribati's easternmost islands, the southern [[Line Islands]] south of [[Hawaii]], have the ''latest''{{efn|'''Latest time on Earth''' here means that calendar day and clock time in this [[time zone]] is ''most'' advanced (or the "''newest''") on Earth and that this place is the ''first'' (''earliest'') on Earth to reach any day and time.}} time on Earth ([[UTC+14:00]] hours).{{r|timeNearIDL1}}

South of Kiribati, the IDL returns westward but remains east of 180°, passing between [[Samoa]] and [[American Samoa]].<ref name=BorneoPost>[http://www.theborneopost.com/2011/06/30/samoa-confirms-dateline-switch/ Samoa confirms dateline switch] {{Webarchive|url=https://web.archive.org/web/20111008180735/http://www.theborneopost.com/2011/06/30/samoa-confirms-dateline-switch/ |date=8 October 2011 }} Borneo Post online. Accessed 11 August 2011.</ref> Accordingly, Samoa, [[Tokelau]], [[Wallis and Futuna]], [[Fiji]], [[Tonga]], [[Tuvalu]], and New Zealand's [[Kermadec Islands]] and [[Chatham Islands]] are all west of the IDL and have the same date. American Samoa, the [[Cook Islands]], [[Niue]], and [[French Polynesia]] are east of the IDL and one day behind.

The IDL then bends southwest to return to 180°. It follows that meridian until reaching [[Antarctica]], which has [[Time in Antarctica|multiple time zones]]. Conventionally, the IDL is not drawn into Antarctica on most maps. (See {{slink||Cartographic practice and convention}} below.)

===Facts dependent on the IDL===
{{refimprove section|date=July 2022}}
According to the clock, the first areas to experience a new day and a [[New Year]] are islands that use UTC+14:00. These include portions of the [[Republic of Kiribati]], including [[Millennium Island]] and [[Kiritimati]] in the [[Line Islands]]. The first major cities to experience a new day are [[Auckland]] and [[Wellington]], New Zealand ([[UTC+12:00]] or [[UTC+13:00]] during daylight saving [[Time in New Zealand|time]]).<ref name=timeNearIDL1>{{Cite web |title=Some Selected Times Around IDL  |url=https://www.timeanddate.com/worldclock/personal.html?cities=3399,724,1890,1033,103,1440,3795,675,282,4019,468,277,22,4679,63,274&wch=2 |website=timeanddate.com |language=en }} Focused mostly on [[List of islands in the Pacific Ocean|Pacific islands]].</ref>

A 1994 realignment of the IDL made Caroline Island one of the first points of land on Earth to reach January 1, 2000, on the calendar (UTC+14:00). As a result, this [[atoll]] was renamed Millennium Island.<ref name="Millennium">{{cite web |work=Pacific Islands Report |title=Kiribati's Caroline Island renamed Millennium Island |date=September 2002 |url=http://www.pireport.org/articles/1997/08/11/kiribatis-caroline-island-renamed-millennium-island |access-date=4 January 2017 |archive-url=https://web.archive.org/web/20190822120549/http://www.pireport.org/articles/1997/08/11/kiribatis-caroline-island-renamed-millennium-island |archive-date=22 August 2019 |url-status=dead }}</ref>

Every day for 2 hours from 10:00 to 12:00 UTC there are 3 different days on earth.   Example: On Tuesday 10:33 UTC it is Monday 22:33 on [[Baker Island]] (US), 23:33 on [[Midway Atoll|Midway]] (US), [[Pago Pago, American Samoa|Pago Pago]] (American Samoa) and [[Alofi]] (Niue), Tuesday almost everywhere else on earth and Wednesday 00:33 in [[Kiritimati]] (Kiribati) in the Line Islands. Then 1 hour 11 minutes later at 11:44 UTC it is Monday 23:44 on Baker Island, Tuesday almost everywhere else on earth, Wednesday 01:44 in Kiritimati and 00:44 in [[Canton Island]] (Kiribati) in the [[Phoenix Islands]], [[Apia]] (Samoa), [[Atafu]] (Tokelau) and [[Nukuʻalofa]] (Tonga) (also in [[Auckland]] during summer when [[Time in New Zealand|NZDT]] is observed). [[Chatham Islands]] (NZ) are also nominally 2 days ahead of Baker Island for 45 minutes in the winter ([[Chatham Standard Time Zone|CHAST]]) and 1 hour 45 minutes in the summer (CHADT).{{r|timeNearIDL1}}

The areas that are the first to see the daylight of a new day vary by the season. Around the [[June solstice]], the first area would be any place within the [[Kamchatka Time|Kamchatka Time Zone]] (UTC+12:00) that is far enough north to experience [[midnight sun]] on the given date. At the [[equinox]]es, the first place to see daylight would be the uninhabited Millennium Island in Kiribati, which is the easternmost land located west of the IDL.{{cn|date=July 2022}}

Near the [[December solstice]], the first places would be [[Research stations in Antarctica|Antarctic research stations]] using [[Time in New Zealand|New Zealand Time]] (UTC+13:00) during summer that experience midnight sun. These include [[Amundsen-Scott South Pole Station]], [[McMurdo Station]], [[Scott Base]] and [[Zucchelli Station]].{{r|timeNearIDL1}}<ref>{{cite web|title=Time in Antarctica|url=https://www.timeanddate.com/worldclock/antarctica|website=timeanddate.com}} Click through to the individual stations' pages. With respect to Scott Base, see [[Ross Dependency]].</ref>