An imaginary line which generally follows the 180° meridian of longitude, one half of the way around the Earth from the Greenwich prime meridian.(Actually, for political and geographic reasons, the International Date Line makes detours around the Aleutian Islands in Alaska and some of New Zealand and actually follows time zone boundaries over some latitude ranges.) The date line marks the boundary between calendar days, so that the date on the New Zealand side is defined to be one day after the date on the Alaska side. Since the line passes down the middle of a time zone (over most latitude ranges), one would switch by an hour at 173.5° W, a day at 180°, and an hour again at 173.5° E.
The construct of a date line is necessary to prevent the continuous accumulation or loss of days as travelers move around the globe as compared to days reckoned by inhabitants remaining in a fixed location. The date line resolves this potential difficulty, but can lead to paradoxes such as a traveler on a long trip from the Asia arriving in the United States several hours before leaving his point of departure.