What line of longitude is the International Date Line based on?

The International Date Line is based on the 180th meridian of longitude. This meridian lies roughly halfway around Earth from the Greenwich prime meridian and runs mainly through the Pacific Ocean.

The line separates adjacent calendar dates: crossing it westward generally advances the date by one day, while crossing it eastward moves the date back by one day. It is not a perfectly straight line, however. The modern date line bends around national territories and island groups so that communities in the same country can share a calendar date.

The 180th meridian is sometimes called the antimeridian because it is opposite the prime meridian. A common misconception is that the International Date Line is fixed by a single international treaty or follows 180° everywhere. In practice, it is a cartographic and timekeeping convention, and its exact course varies, especially around Russia, the Aleutian Islands, and Kiribati.