Editing International Date Line (section)

== De facto and de jure date lines ==
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There are two ways [[time zone]]s and thereby the location of the International Date Line are determined: one on land and adjacent [[territorial waters]], and the other on open seas.

All nations unilaterally determine their standard [[time zone]]s, applicable only on land and adjacent territorial waters. This date line can be called ''[[de facto]]'' since it is not based on international law, but on national laws. These national zones do not extend into international waters.

The [[nautical date line]], not the same as the IDL, is a ''[[de jure]]'' construction determined by international agreement. It is the result of the 1917 [[Anglo-French Conference on Time-keeping at Sea]], which recommended that all ships, both military and civilian, adopt hourly standard time zones on the high seas. The United States adopted its recommendation for U.S. military and merchant marine ships in 1920. This date line is implied but not explicitly drawn on time zone maps. It follows the [[180th meridian|180° meridian]] except where it is interrupted by territorial waters adjacent to land, forming gaps—it is a pole-to-pole dashed line. The 15° [[Gore (segment)|gore]] that is offset from UTC by 12 hours is bisected by the nautical date line into two 7.5° gores that differ from UTC by ±12 hours.

In theory, ships are supposed to adopt the standard time of a country if they are within its territorial waters within {{convert|12|nmi|mi km}} of land, then revert to international time zones (15° wide pole-to-pole [[Gore (segment)|gore]]s) as soon as they leave. In practice, ships use these time zones only for radio communication and similar purposes. For internal (within-ship) purposes, such as work and meal hours, ships use a time zone of their own choosing.

===Cartographic practice and convention===
The IDL on the map in this article and all other maps is based on the ''de facto'' line and is an artificial construct of cartographers, as the precise course of the line in international waters is arbitrary. The IDL does not extend into Antarctica on the world time zone maps by the United States [[Central Intelligence Agency]] (CIA)<ref name=CIA>{{cite web |url=https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf |title=Standard Time Zones of the World by the CIA |access-date=23 September 2014 |archive-date=8 January 2018 |archive-url=https://web.archive.org/web/20180108181645/https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/physical/pdf/standard_time_zones_of_the_world.pdf |url-status=dead }}</ref> or the United Kingdom's [[HM Nautical Almanac Office|His Majesty's Nautical Almanac Office]] (HMNAO).<ref name=HMNAO>{{cite web |url=http://astro.ukho.gov.uk/nao/miscellanea/WMTZ/Wmtz160224.pdf |title=Standard Time Zones by HM Nautical Almanac Office |publisher=HM Nautical Almanac Office and US Naval Observatory (jointly) |access-date=20 May 2016 |archive-date=11 June 2016 |archive-url=https://web.archive.org/web/20160611172910/http://astro.ukho.gov.uk/nao/miscellanea/WMTZ/Wmtz160224.pdf |url-status=dead }} {{selfref inline|Note that this map uses the opposite sign convention for time zones from that used in Wikipedia.}}</ref> The IDL on modern CIA maps now reflects the most recent shifts in the IDL.<ref name=CIA /> {{xref|(see: {{slink||Historic alterations}})}} The current HMNAO map does not draw the IDL in conformity with recent shifts in the IDL; it draws a line virtually identical to that adopted by the UK's Hydrographic Office about 1900.<ref>A. M. W. Downing, [http://articles.adsabs.harvard.edu/full/1900JBAA...10..176D "Where the day changes"] {{Webarchive|url=https://web.archive.org/web/20170721110851/http://articles.adsabs.harvard.edu/full/1900JBAA...10..176D |date=21 July 2017 }}, ''Journal of the British Astronomical Association'', vol x, no 4, 1906, pp. 176–178.</ref> Instead, HMNAO labels island groups with their time zones, which do reflect the most recent IDL shifts.<ref name=HMNAO /> This approach is consistent with the principle of national and nautical time zones: the islands of eastern Kiribati are actually "islands" of Asian date (west side of IDL) in a sea of American date (east side of IDL). Similarly, the western Aleutian Islands are islands of American date in a sea of Asian date.

No international organization, nor any treaty between nations, has fixed the IDL drawn by cartographers: the 1884 [[International Meridian Conference]] explicitly refused to propose or agree to any time zones, stating that they were outside its purview. The conference resolved that the Universal Day, midnight-to-midnight [[Greenwich Mean Time]] (now redefined and updated as [[Coordinated Universal Time|Coordinated Universal Time, or UTC]]), which it did agree to, "shall not interfere with the use of local or standard time where desirable".<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|page=134|access-date=20 May 2016|year=1884|archive-date=11 September 2015|archive-url=https://web.archive.org/web/20150911112200/http://www.gutenberg.org/files/17759/17759-h/17759-h.htm|url-status=live}} Quote is from the session of 14 October.</ref> From this comes the utility and importance of UTC or "Z" ("Zulu") time: it permits a single universal reference for time that is valid for all points on the globe at the same moment.