National Geographic Society. 1992. National Geographic atlas of the world. Revised 6th ed. Washington, D.C. USA: National Geographic Society.
Background: The International Date Line (IDL), also known as the Date Line, is an imaginary line on the surface of the Earth opposite the Prime Meridian (0°) that offsets the date as one travels east or west across it. Roughly along 180° longitude with diversions to pass around some territories and island groups, it corresponds to the time zone boundary separating +12 and -12 hours GMT (UTC). Crossing the IDL traveling east results in a day (24 hours) being subtracted, while crossing west results in a day being added.
Note: The precise location of the IDL is not fixed by any international law, treaty, or agreement. As a result, various maps and atlases will display it differently (see some examples).