The 'international date line is
|“||[t]he line coinciding approximately with the antimeridian of Greenwich, modified to avoid certain habitable land. In crossing this line there is a date change of one day.||”|
"The time zone east of the international date line is twelve hours ahead of Greenwich mean time; the time zone west of the international date line is twelve hours behind Greenwich mean time. A traveler going west across the date line adds a day; a traveler going east across it subtracts a day."