LEWIS CARROLL poses a question, and indeed provides the answer, in The Hunting of the Snark:
What's the good of North
Poles and Equators,
Tropics, Zones and Meridian
Lines?
So the Bellman would cry:
and the crew would reply,
They are merely conven-
tional signs.
One important conventional feature of the globe not on the Bellman's list is an imaginary zig-zag line that staggers drunkenly down the middle of the Pacific Ocean. It is called the International Date Line, and its purpose is very clear indeed: it is a chronological barrier to protect Tuesdays from inconvenient union with any neighbouring Wednesdays.
The need for a date line became apparent at the International Meridian Conference in 1884, at which Greenwich Mean Time was formally adopted as the bench-mark from which all other standard times throughout the world were measured. By this arrangement, the world is divided into 24 time zones of 15 of longitude, corresponding to the 24 hours of the day. Moving east from Greenwich you add an hour every 15 degrees, and moving west you subtract an hour in the same way.
Now if you carry this arrangement to its logical conclusion, a person travelling right around the world will either gain or lose an entire day, depending on the direction of travel. Obviously someone had to draw the line, and draw it they did down the Pacific Ocean, opposite the "prime" Greenwich meridian, at 180 degrees either east or west. When crossing the date line you not only change the hour, as when moving from one time zone to another, but the date as well. Moving from east to west, the date becomes one day later August 24th instantly becoming August 25th; crossing eastwards, date changes to a day earlier.
The date line does not follow the 180" meridian exactly, but deviates here and there to avoid land areas, or to avoid splitting the islands of an archipelago, thereby keeping confusion in the affairs of everyday life in Oceania to a minimum. Heading south from the north pole along 180, it takes a sudden lurch to the east through the Bering Strait to avoid separating the Chukotsky Peninsula from the rest of Russia. Then it veers for a time an equal distance to the west, to include the Aleutian Islands with Alaska. South of the equator, the line veers to the west again, but past New Zealand it reverts to the 180" marker, and then proceeds in an orderly fashion in the direction of the south pole.