International Date Line
What is the International Date Line?
The International Date Line is an imaginary boundary, mostly through the Pacific Ocean, where the calendar date changes. Crossing westward usually moves you to the next date; crossing eastward moves you to the previous one.
The forced cut
If you fly west around the world, you keep setting your watch back. When you get home, a day has gone missing unless you add one somewhere. That somewhere is this line.
It kinks around Kiribati, around eastern Russia, around a few islands that picked a side. The kinks are politics. The need for a seam is geometry.
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Two cities, two dates
Open the world clock and find a pair — Auckland and Honolulu usually work. Same instant, different date. There is no moment when the whole world shares one calendar day.