Time zones
How does the International Date Line work?
The place where neighbouring civil time zones can carry dates one calendar day apart.
A date boundary, not a wall
The International Date Line is the broad boundary where civil dates change as you travel east or west across the Pacific. Crossing westward normally adds a day; crossing eastward normally subtracts one.
Why it bends
The line roughly follows the 180-degree meridian but bends around countries and island groups so that a jurisdiction can share one civil date. It is a convention assembled from time-zone choices, not a single treaty-defined physical line.
The same instant, different dates
Auckland can be on Thursday while Honolulu is still on Wednesday, even though both clocks describe the same instant. Conversion must therefore show the date alongside the hour whenever a route approaches or crosses the Pacific date boundary.