International Date Line


The International Date Line 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. It then passes through the Bering Strait between the Diomede Islands at a distance of 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 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, have the earliest time on Earth.
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.
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.

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.
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. 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, 23:33 on Midway, Pago Pago and Alofi, Tuesday almost everywhere else on earth and Wednesday 00:33 in Kiritimati 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 in the Phoenix Islands, Apia, Atafu and Nukuʻalofa . Chatham Islands are also nominally 2 days ahead of Baker Island for 45 minutes in the winter and 1 hour 45 minutes in the summer.
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 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 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

There are two ways time zones 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 zones, 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 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 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 of land, then revert to international time zones as soon as they leave. In practice, ships use these time zones only for radio communication and similar purposes. For internal 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 or the United Kingdom's His Majesty's Nautical Almanac Office. The IDL on modern CIA maps now reflects the most recent shifts in the IDL. 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. Instead, HMNAO labels island groups with their time zones, which do reflect the most recent IDL shifts. This approach is consistent with the principle of national and nautical time zones: the islands of eastern Kiribati are actually "islands" of Asian date in a sea of American date. 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, which it did agree to, "shall not interfere with the use of local or standard time where desirable". From this comes the utility and importance of UTC or "Z" time: it permits a single universal reference for time that is valid for all points on the globe at the same moment.

History

The 14th-century Arab geographer Abulfeda predicted that circumnavigators would accumulate a one-day offset to the local date. This phenomenon was confirmed in 1522 at the end of the 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. 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. Cardinal Gasparo Contarini, the Venetian ambassador to Spain, was the first European to give a correct explanation of the discrepancy.

Historic alterations

Philippines (1521 and 1844)

claimed the Philippines for Spain on Saturday,, having sailed westward from Seville across the Atlantic Ocean and the Pacific Ocean. As part of New Spain, the Philippines had its most important communication with Acapulco in Mexico, so it was on the eastern side of the IDL despite being on the western edge of the Pacific Ocean. As a result, the Philippines was one day behind its Asian neighbours for 323 years, 9 months and 2 days from Saturday, 16 March 1521 ' until Monday, 30 December 1844 '.
After Mexico gained its independence from Spain on 27 September 1821, Philippine trade interests turned to Imperial China, the Dutch East Indies and adjacent areas, so the Philippines decided to join its Asian neighbours on the west side of the IDL. To advance the calendar by one day, on 16 August 1844 the then governor-general Narciso Claveria, ordered that Tuesday, should be removed from the calendar. Monday, was followed immediately by Wednesday,. The change also applied to the other remaining Spanish colonies in the Pacific: the Caroline Islands, Guam, Mariana Islands, Marshall Islands and Palau as part of the Captaincy General of the Philippines. European publications were generally unaware of this change until the early 1890s, so they erroneously gave the International Date Line a large western bulge for the next half century.