International Date Line

International Date Line, a line running mainly along the longitude of 180° but with adjustments to avoid the division of certain Island groups which lie astride it, the Aleutian Islands in the north and the Fiji, Tonga, and Kermadoc groups, with New Zealand, in the south. It is on this international dateline that the zone times of +12 hours and -12 hours meet and the date changes. If a traveller round the world sets out from Greenwich (see greenwich mean time), on 0° longitude, and travels eastwards, he puts his clock forward by one hour as he crosses each 15° of longitude, so that by the time he reaches Greenwich again, his clock has been put forward by 24 hours and he is one day ahead in date. To correct this anomaly, he subtracts a day when he crosses the international dateline eastwards and adds a day when he crosses it westwards. So in the former case the same calendar date is used for two successive periods of 24 hours; in the latter case one calendar date is omitted altogether.