International Date Line
International Date Line, imaginary line that runs down the middle of the Pacific Ocean and mostly follows the 180th meridian. Its purpose is to mark the spot on the earth's surface where each new calendar day begins. Clocks are set 1 hour earlier for every 15° (1/24 of the 360° circle) westward around the globe. A full circuit would move back the calendar as well as the clock. It has thus been necessary, particularly with the advent of rapid global communication and travel, to institute an arbitrary date line. When Tuesday is dawning just east of this line, Wednesday is dawning just west. Thus Japan is one of the first countries to reach the new day, while the United States is one of the last.