International Date Line
From Memory Alpha, the free Star Trek reference
The International Date Line was a geographic boundary on Earth that existed during the 20th century. 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")
Although never mentioned by name, the International Date Line can clearly be seen on the map of the Pacific that the Talosians download from the USS Enterprise computer in 2254.
