The International Date Line (IDL) is an imaginary longitude line that runs through the Pacific Ocean. It is located at about 180 degrees east or west of the Greenwich Meridian.
The IDL is deliberately placed so that it does not run through the middle of any country. As a result, it does not follow the 180-degree meridian in a straight line. The time difference on each side of the IDL is not always 24 hours, because there may be local time regulations in effect, but generally, if one crosses the IDL to the east, a day is subtracted, and if one crosses it to the west, a day is added.