International Date Line (IDL)

A conventional boundary near 180° longitude across which the civil calendar date changes by one day.

It bends around countries and island groups so political territories can share a chosen date, and it is not defined by a single universal treaty line. Crossing westward normally advances the date by one day; crossing eastward sets it back by one day.

The date change is separate from adjusting clock time for a time zone. Navigators should use the vessel’s prescribed timekeeping and logbook procedure and verify the legal time used by the destination.