The International Date Line is exactly defined by the anti-meridian.

The International Date Line (IDL) is an imaginary line of navigation on the surface of the Earth that runs from the north pole to the south pole and demarcates the change of one calendar day to the next). The anti-meridian is the meridian which is 180° east or west of the Prime Meridian with which it forms a great circle. It is common to both east longitude and west longitude. It is used as the basis for the IDL because for the most part it passes through the open waters of the Pacific Ocean; but the IDL deviates from the antimeridian to pass around some territories and island groups. This picture shows the deviation:

For example, the Aleutian Islands, part of Alaska, lie across the antimeridian, but wholly to the east of the IDL. So, although the whole of Alaska is always on the same calendar day as the rest of the USA, part of the state lies in the extreme west of the western hemisphere and part of it in the extreme east of the eastern hemisphere!