The International Date Line! The International Date Line (IDL) is a generally north-south imaginary line on the surface of the Earth, passing through the middle of the Pacific Ocean, that designates the place where each calendar day begins. It is roughly along 180° longitude, opposite the Prime Meridian, but it is drawn with diversions to pass around some territories and island groups. Crossing the IDL travelling east results in a day or 24 hours being subtracted, so that the traveller repeats the date to the west of the line. Crossing west results in a day being added, that is, the date is the eastern side date plus one calendar day. The line is necessary in order to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances. Geography For part of its length, the International Date Line follows the meridian of 180° longitude, roughly down the middle of the Pacific Ocean. To avoid crossing nations internally, however, the line deviates to pass around the far east of Russia and various island groups in the Pacific. In the north, the date line swings to the east of Wrangel island and the Chukchi Peninsula and through the Bering Strait passing between the Diomede Islands at a distance of 1.5 km (1 mi) from each island. It then goes southwest, passing west of St. Lawrence Island and St. Matthew Island, until it passes midway between the United States' Aleutian Islands and Russia's Commander Islands before returning southeast to 180°. This keeps Russia which is north and west of the Bering Sea and the United States' Alaska which is east and south of the Bering Sea, on opposite sides of the line in agreement with the date in the rest of those countries. Two uninhabited atolls, Howland Island and Baker Island, just north of the equator in the central Pacific Ocean, have the oldest time on Earth of UTC"12:00 hours, along with ships at sea between 172.5°W and 180°. The date line then circumscribes the country of Kiribati by swinging far to the east, almost reaching the 150° meridian. Kiribati's easternmost islands, the southern Line Islands south of Hawaii, have the most advanced time on Earth of UTC+14 hours. In the South Pacific, the date line also lies east of 180°, so that Wallis and Futuna, Fiji, Tonga, and New Zealand's Kermadec Islands and Chatham Islands have the same date, but Samoa and New Zealand's Cook Islands are one day earlier. Samoa plans to adjust the dateline on December 29, 2011. [1] 1 / 3 The International Date Line! If one flies round the world from east to west (the same direction as Magellan), one hour is lost for every 15° of longitude crossed, losing 24 hours for one circuit of the globe; one compensates by adding 24 hours when crossing the International Date Line (also from east to west). The International Date Line must therefore be observed in conjunction with the Earth's time zones: on crossing it, in either direction, the calendar date is adjusted by one day. For two hours every day, between 10:00 and 11:59 (UTC), there are actually three different days observed at the same time in different places. At UTC time Thursday 10:15, for example, it is Wednesday 23:15 in Samoa, which is eleven hours behind UTC, and it is Friday 00:15 in Kiritimati (separated from Samoa by the IDL), which is 14 hours ahead of UTC. For the first hour (UTC 10:00–10:59), this is true for inhabited territories, whereas during the second hour (UTC 11:00–11:59) it is only true if one counts an uninhabited maritime time zone twelve hours behind UTC. The IDL drawn on the map on this page and all other maps is now and always has been an artificial construct of cartographers—the precise course of the line in international waters is arbitrary. No international organization nor any treaty between nations has fixed the straight line segments and their junctions. All nations unilaterally determine their standard time zones, which are applicable only on land and adjacent territorial waters. These national zones do not extend into international waters. Indeed, 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 "Zulu" time: it permits a single and universal reference for time that is valid for all points on the globe at the same moment. The nautical date line 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, for example, 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. Ships should adopt the standard time of a country if they are within its territorial waters, but should revert to international time zones (15° wide pole-to-pole gores) as soon as they leave its territorial waters. In reality they use these time 2 / 3 The International Date Line! zones only for purposes such as radio communication. For internal ship's purposes, e.g. for work and meal hours, they use a time zone of their own choosing. The 15° gore that is offset from UTC by twelve hours is bisected by the nautical date line into two 7.5° gores that differ from UTC by ±12 hours. http://en.wikipedia.org/wiki/International_Date_Line 3 / 3.