History
The International Date Line: history, rules and exceptions
The Date Line separates consecutive calendar dates, but it is a convention that bends around political choices rather than a legal wall at exactly 180 degrees.
A globe needs somewhere for the date to change
If local times progress around the Earth, the calendar must step from one date to the next somewhere. The International Date Line roughly follows the 180-degree meridian in the Pacific, opposite Greenwich.
Crossing changes the date label
Cross westward and the calendar normally advances by one day; cross eastward and it normally goes back by one day. The instant itself does not change—only the civil date used to describe it.
Countries can bend the line
The Date Line has no legal international status. It zigzags around political borders and island groups because jurisdictions choose the date they observe. That is why a time conversion across the Pacific must show both the hour and the date.