A line of longitude generally 180 degrees east and west of the prime meridian. The date is one day earlier to the east of the line.

The International Date Line (IDL) is an imaginary north-south line (at the 180th meridian), in the Pacific Ocean, at which the date changes. The east side of the IDL is a calendar day earlier than the west side. The actual IDL used is not a straight line, but zigzags around certain populated areas.