(1) the equator from south to north Explanation: A traveler crossing the International Date Line eastbound subtracts one or 24 hours, so that the calendar date to the west of the line is repeated. Crossing the IDL westbound results in 24 hours being added, advancing the calendar date by one day, The International Date Line is necessary to have a fixed, albeit arbitrary, boundary on the globe where the calendar date advances in the westbound direction.