<P> 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 . </P> <P> 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 . </P> <P> Ships are supposed to adopt the standard time of a country if they are within its territorial waters within 12 nautical miles (14 mi; 22 km) of land, then revert to international time zones (15 ° wide pole - to - pole gores) as soon as they leave . In reality, 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 . </P> <P> The IDL on the map on this page 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 (CIA) or the United Kingdom's Her Majesty's Nautical Almanac Office (HMNAO). The IDL on modern CIA maps now reflects the most recent shifts in the IDL (see § Historical alterations below). 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 (west side of IDL) in a sea of American date (east side of IDL). </P>

Where do the prime meridian and the international dateline meet