International Date Line
imaginary line that demarcates the change of one calendar day to the next / From Wikipedia, the free encyclopedia
The International Date Line (IDL), also known as just the Date Line, is an imaginary line on the surface of the Earth, going from north to south in the Pacific Ocean. The date becomes one day later as one travels across it in western direction, and one day earlier as one travels across it in eastern direction.
The reason for this effect is that the countries on the western side of the International Date Line, (in or outside eastern Asia) have the time zone 10–12 hours more than Greenwich. And the countries on eastern side of it (Alaska/Hawaii and other areas) have the time zone 9–12 hours less than Greenwich. So when travelling across the line, one's watch has to be adjusted 20–24 hours, depending on the time zones.
For example, New Zealand is twelve hours ahead of Greenwich Mean Time while Hawaii is ten hours behind Greenwich Mean Time. When travelling from New Zealand to Hawaii the clocks must be switched 22 hours backwards, about one day.