International Date Line - de Facto and De Jure Date Lines

De Facto and De Jure Date Lines

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. No international organization, nor any treaty between nations, has fixed the straight line segments and their junctions of the International Date Line drawn by cartographers. 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 (now known as Coordinated Universal Time, or UTC), 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, which is not the same as the International 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. 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.

Ships should adopt the standard time of a country if they are within its territorial waters, within 12 nautical miles of land (about 22 km or 14 miles), but should revert to international time zones (15° wide pole-to-pole gores) as soon as they leave territorial waters. In reality, ships use these time zones only for radio communication and similar purposes. For internal purposes, such as work and meal hours, passenger events, and facilities opening hours, ships use a time zone of their own choosing.

The International Date Line 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 cartographer's 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 (CIA) or the United Kingdom's Her Majesty's Nautical Almanac Office (HMNAO) . The International Date Line drawn on the modern CIA and HMNAO maps, which ignores Kiribati's 1995 shift, is virtually identical to that adopted by the UK's Hydrographic Office about 1900.