The International Date Line (IDL) is an imaginary line on the surface of the Earth opposite the Prime Meridian, where the date changes as one travels across east or west. Roughly along 180° longitude, it almost corresponds to the time zone boundary separating -12 and +12 hours Greenwich Mean Time (GMT). Crossing the IDL, travelling east results in a day or 24 hours being added and crossing west results in a day being subtracted. The exact number of hours depends on the time zones.