International Date Line

International Date Line, irregular line drawn on the map of the Pacific Ocean, near, and in many places coincident with, the 180th meridian. It marks the place where navigators change their date by one day on a transpacific voyage. East of the line it is one day earlier than to the west.

Any traveller circling the globe in a westwards direction lengthens the day by one hour for every 15° of longitude travelled because the traveller is following the apparent motion of the Sun; by the time he or she has travelled completely around the world, the traveller is one full day ahead of the people who have remained at the starting place of the trip. Similarly, going eastwards, a traveller arrives a day behind.

Close to the 180th meridian, nearly in the middle of the Pacific Ocean (a place chosen because of the virtual absence of inhabited land in the region), navigators going westwards add a day to their calendars (for example, the day after August 6 would be August 8), and navigators going eastwards drop a day from their calendars (for example, the day after August 6 would be August 6 again) to correct for this gain or loss of time. The date line is curved eastwards around Siberia, westwards around the Aleutian Islands, and eastwards around the Fiji Islands and New Zealand in order to avoid crossing land.

The date line was instituted as part of Standard Time, the system of time zones proposed by the Canadian engineer Sandford Fleming and enacted by international agreement in 1885.