Because when you cross it, you must change your calendar by one day,
otherwise you will be out of sync with everyone else in the world.
The Pacific one.
The International Date line has most of its length in the Pacific Ocean. The line extends from the Arctic Ocean to the Southern Ocean roughly along 180° longitude (E or W).
the international date line
Actually, it is in the name and pretty simple. It is surprisingly called the International Date Line
The International Date Line is an imaginary line on the Earth's surface that marks the change in calendar day. When you cross this line from east to west, you either add or subtract a day depending on the direction. It runs roughly along the 180° meridian in the Pacific Ocean.
the whole world agrees that the date changes on each side of the line
International Date Line
The line 180 degrees from the Prime Meridian is called the International Date Line, which serves as the boundary between one day and the next on the Earth's surface.
Litmatch
The International Date Line.
The line 180 degrees east and west of the prime meridian is called the International Date Line.
Its called the International date line. See the related link for more information.