What is the International Date Line?

Located at 0 degrees longitude, the prime meridian passes through Greenwich, England. Halfway around the world in the middle of the Pacific Ocean (180 degrees from Greenwich) is the International Date Line (IDL), where the date changes across the boundary of the time zone.

The entire world is on the same date only at the instant when it is noon in Greenwich, England, and midnight at the IDL. At all other times, there are different dates on each side of the IDL.