International Date Line Wiki

Jagged red line down the right is the International Date Line

The International Date Line (IDL), also known as just the Date Line, is an imaginary line on the surface of the Earth, going from north to south in the Pacific Ocean. The date becomes one day later as one travels across it in western direction, and one day earlier as one travels across it in eastern direction.

The reason for this effect is that the countries on the eastern side of the International Date Line, (in or outside eastern Asia) have the time zone 10–12 hours more than Greenwich. And the countries on western side of it (Alaska/Hawaii and other areas) have the time zone 9–12 hours less than Greenwich. So when travelling across the line, one's watch has to be adjusted 20–24 hours, depending on the time zones.

For example, New Zealand is twelve hours ahead of Greenwich Mean Time while Hawaii is ten hours behind Greenwich Mean Time. When travelling from New Zealand to Hawaii the clocks must be switched 22 hours backwards, about one day.

International Date Line follows the meridian of 180° longitude down the middle of the Pacific Ocean for some time. So that it does not cross nations, it passes around the far east of Russia and other archipelagos in the Pacific.

In the north the date line turns to the east through the Bering Strait and then west past the Aleutian Islands in order to keep Alaska and Russia on opposite sides of the line. This is to keep it in agreement with the date of the rest of those countries. The date line passes equidistantly between the two Diomede Islands—Little Diomede Island (US) and Big Diomede Island (Russia)—at a distance of 1.5 km (1 mi) from each island.

The date line circumvents the territory of Kiribati by swinging far to the east, almost reaching the 150° meridian.

In the South Pacific the date line swings east such that Wallis and Futuna, Fiji, Tonga, and New Zealand's Kermadec Islands have the same date but Samoa is one day earlier.