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Related to Date Line: deadline, International Date Line
an arbitrary line drawn on the earth’s surface to demarcate places where the time of day is the same but the date differs by one full day. The date line coincides with the 180th meridian for most of its length, departing in places to avoid passing over dry land. East of the date line it is one day earlier than it is to the west. The date line serves to maintain a correct count of days of the month when traveling.
The traveler moving eastward passes through points where clocks indicating local (or zone) time show an ever-increasing time reading as compared to the local time at the starting point. By gradually setting his watch ahead, the traveler will have gained a day at the end of a journey around the world. Conversely, if the traveler makes his journey around the world from east to west he will have lost a day at the end of the journey. To resolve this problem, ships or airplanes crossing the date line from west to east set the date back by one day. For instance, if they approach the date line at ten o’clock on May 2, after crossing the line they will assume that it is ten o’clock on May 1. In traveling from east to west one day is added to the calendar date, so that if the date line is approached at ten o’clock on May 2, the time is assumed to be ten o’clock on May 3 after the date line has been crossed.