The International Date Line was a geographic boundary on Earth. The line ran along a degree of longitude in the Pacific Ocean and was used for timekeeping purposes; specifically, the boundary determined where one day began and ended in terms of the various time zones which were then used on Earth long before the advent of stardates.
The American states of Hawaii and Alaska were relatively close to the International Date Line and the line ran directly through the middle of the Bering Sea. The former Earth nation of the Soviet Union also had territory very close to the date line. (TOS: "The Cage")