Discussion of the International Date Line

The International Date Line (IDL) is an internationally accepted demarcation of the surface of Earth, running between the South and North Poles and serving as the boundary between one calendar day and the next. It passes through the Pacific Ocean, roughly following the 180° 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.

Example depicting situation at 04:00 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 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.

Failing to do this would make their time inaccurate to the local time.

The Arab geographer Abulfeda (1273–1331) 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 circumnavigation (1519–1522), 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.  Cardinal Gasparo Contarini, the Venetian ambassador to Spain, was the first European to give a correct explanation of the discrepancy.

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/or 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.

The IDL remains on the 180° meridian until passing the equator. Two US-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 westwards 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.

          https://en.wikipedia.org/wiki/International_Date_Line