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 of the Earth that lies opposite 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 there is one day later than what it truly is.

Anyone travelling west and passing the line must add a day to what they would otherwise expect the date and time to be. Correspondingly, those going east must subtract a day, as Verne's travellers neglected to.