Each place has its own local sun time, but there are obvious difficulties in having local times which differ by fractions of an hour. For convenience, therefore, the world is divided into 24 time zones of 15 degrees of longitude corresponding to the 24 hours of the day.
These are closely adhered to by ships at sea, but on land the boundaries of the zones are altered to coincide as much as possible with political or administrative divisions. And - as will happen early tomorrow morning - if it is decided to choose some different time within a time zone, everyone must change together, or chaos will ensue.
But if you think about it, you will realise that all these hourly increments of time, side by side, encircling the globe, cannot go on for ever. Anyone travelling in a westerly direction must subtract an hour every 15 degrees, but if this arrangement were carried to its ultimate conclusion, someone travelling right around the world would end up back where he or she started, but on the previous day.
Obviously, the line had to be drawn, and drawn it was, opposite the "prime" Greenwich meridian, at 180 degrees either east or west.
When crossing the International Date Line, as it is called, you not only change the hour, as you would when moving from one time zone to another, but the date as well. Moving from east to west, the date becomes one day later, March 27th instantly becoming March 28th; crossing eastwards, you must retard your calendar by a day.
Now, "if there are obstacles," wrote Bertolt Brecht, "the shortest distance between two points may be the crooked line". And so it is with the International Date Line. This chronological barrier, protecting Tuesdays from inconvenient union with any neighbouring Wednesdays, staggers drunkenly down the middle of the Pacific Ocean.
The Line does not follow the 180 degree meridian exactly, but deviates here and there to avoid land areas, or to avoid splitting the islands of an archipelago, thereby minimising confusion in everyday affairs in Oceania.
Heading south from the North Pole along 180 degrees, 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 its 180 degree marker, and then proceeds in an orderly fashion towards the South Pole.