The International Date Line (IDL) is an imaginary line on the surface of the Earth opposite the Prime Meridian.
The IDL is where the date changes as one travels east or west across it. Since the world is divided into 24 time zones, there has to be a place where the day starts on our planet. The IDL along the 180° line of longitude, exactly one-half way around the planet from Greenwich, serves just that purpose. Cross the line from the east to the west (example: Alaska to Japan) and a day is added (“losing” a day). Cross from west to the east (example: Japan to Alaska) and a day is subtracted (“gaining” a day). The exact number of hours lost or gained depends on the time zones.
Without the International Date Line, people travelling west around the globe would discover upon return home that it would seem as though an extra day has passed. This situation actually happened to Magellan’s crew when they returned home after their circumnavigation of the earth.