Why does it go back 1 day from west to east through the meridian of 180 degrees? and why does it add 1 day from east to west across the meridian of 180 degrees?

The Earth's longitude measures 360, so the halfway point from the prime meridian is the 180 longitude line. The meridian at 180 longitude is commonly known as the International Date Line. As you pass the International Date Line, you either add a day (going west) or subtract a day (going east.)

Explanation:

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