International date line

The international date line is an imaginary line that for the most part is on the ±180° Longitude, but has an odd shape to pass around Russia and islands in the Pacific. It is on the side opposite to the prime meridian.

Warning: Wikipedia contains spoilers

Its purpose is to offset the hour that is added as one travels east through successive time zones. The effect of ignoring the date line is seen in Jules Verne's novel Around the World in Eighty Days, in which the travellers return to London after a trip around the world, thinking the date is one day ahead of what it truly is.

If you are travelling west and pass the line you add a day on. Correspondingly, if you are going east you subtract a day, as Verne's travellers neglected to.