The International Date Line (IDL) is the line extending between the South and North Poles that is the boundary between one calendar day and the next. It passes through the Pacific Ocean, roughly following the 180.0° line of longitude and deviating to pass around some territories and island groups. Crossing the date line eastbound decreases the date by one day, while crossing the date line westbound increases the date. The line is a cartographic convention and is not defined by international law. This has made it difficult for cartographers to agree on its precise course and has allowed countries through whose waters it passes to move it at times for their convenience.
Geography
Circumnavigating the globe People traveling westward 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.
Description
The IDL is roughly based on the 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 Greenwich prime meridian running through the 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.) It then passes through the Bering Strait between the Diomede Islands at a distance of 1.5 kilometres (0.93 mi) from each island at 168°58′37″ W. 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 time on Earth (UTC−12:00 hours). The IDL circumscribes Kiribati by swinging far to the east, almost reaching the 150°W meridian. Kiribati's easternmost islands, the southern Line Islands south of Hawaii, have the latest time on Earth (UTC+14:00 hours). South of Kiribati, the IDL returns westward but remains east of 180°, passing between Samoa and American Samoa. 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 multiple time zones. Conventionally, the IDL is not drawn into Antarctica on most maps. (See § Cartographic practice and convention below.)
Facts dependent on the IDL 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). A 1994 realignment of the IDL made Caroline Island one of the first points of land on Earth to reach 1 January 2000, on the calendar (UTC+14:00). As a result, this atoll was renamed Millennium Island. 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 (US), 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 NZDT is observed). Chatham Islands (NZ) are also nominally 2 days ahead of Baker Island for 45 minutes in the winter (CHAST) and 1 hour 45 minutes in the summer (CHADT). 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 Zone (UTC+12:00) that is far enough north to experience midnight sun on the given date. At the equinoxes, the first place to see daylight would be the uninhabited Millennium Island in Kiribati, which is the easternmost land located west of the IDL. Near the December solstice, the first places would be Antarctic research stations using 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.
De facto and de jure date lines
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