<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> <P> No international organization, nor any treaty between nations, has fixed the IDL drawn by cartographers: 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 "Z (Zulu)" time: it permits a single universal reference for time that is valid for all points on the globe at the same moment . </P> <P> As part of New Spain, the Philippines long had its most important communication with Acapulco in Mexico and was accordingly on the east side of the IDL despite being at the far western edge of the Pacific Ocean . From 1521 to 1844, the Philippines was one day behind its Asian neighbors (1521 was the first European visit, when Ferdinand Magellan claimed the area for Spain . Colonization began in 1565). After Mexico gained its independence from Spain in 1821, Philippine trade interests turned to Imperial China, the Dutch East Indies and adjacent areas, so the Philippines decided to shift back to the west side of the IDL by removing Tuesday, 31 December 1844 from its calendar . The change was also applied to Mariana Islands, Guam and Caroline Islands since they also belonged to Spain . Because of this change, New Year's Eve happened on Monday, 30 December 1844 . Effective Wednesday, 1 January 1845, the Philippines, Guam, Mariana Islands and Caroline Islands switched back to Asian date . Western publications were generally unaware of this change until the early 1890s, so erroneously gave the International Date Line a large western bulge for the next half century . </P>

Where is the international date line located on a map