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 traveler circling the globe in a westward direction lengthens the day by 1 hour for every 15° of longitude traveled because the traveler is following the apparent motion of the sun; by the time he or she has traveled completely around the world, the traveler is one full day ahead of the people who have remained at the starting place of the trip. Similarly, going eastward, a traveler 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 land and of civilization in the region, navigators going westward add a day to their calendars (for example, the day after August 6 would be August 8), and navigators going eastward drop a day from their calendars (for example, the day after August 6 would be August 6) to correct for this gain or loss of time. The date line is curved eastward around Siberia, westward around the Aleutian Islands, and eastward around the Fiji Islands and New Zealand; the line is so curved to avoid crossing land.