Which meridian is used as the main reference for the International Date Line?

The 180th meridian is the main geographic reference for the International Date Line.

The International Date Line is an imaginary boundary where the calendar date changes when a traveller crosses it. It generally follows the 180th meridian, which lies halfway around Earth from the Prime Meridian at Greenwich, but it bends around countries and island groups to avoid splitting communities into different calendar dates.

The line is not an international treaty boundary and does not run perfectly straight. Its largest deviations occur in the Pacific, including a major eastward bend around Kiribati. In 1995, Kiribati moved the date line so all of its inhabited islands would share the same calendar day.

Crossing the line westward advances the calendar by one day; crossing eastward moves it back by one day. This is separate from ordinary time-zone changes, which alter clock time but do not necessarily change the date. The Prime Meridian remains the reference for longitude, not the date line.