International date line，imaginary line，on the earth‘s surface, generally following the 180 degree meridian of longitude, by international agreement, travellers change dates.
The date line is necessary to avoid a confusion that would otherwise result.
For example, if an airplane were to travel westward with the sun, 24hr would elapse as it circled the globe, but it would still be the same day for those in the airplane while it would be one day later for those on the ground below them.
The same problem would arise if two travellers journeyed in opposite directions to a point on the opposite side of the earth—180 degree of longitude distant.
The apparent paradox is resolved by requiring that the traveller crossing the date line change his date, thus bringing the travellers into agreement when they meet.