International Date Line
If someone travels around the globe in an airplane from east to west (the same direction as Magellan), they should subtract one hour for every 15° of longitude crossed, losing 24 hours for one circuit of the globe; but 24 hours are added when crossing the International Date Line (from east to west). The International Date Line must therefore be observed in conjunction with Earth's time zones: the net adjustment to one's watch is zero. If one crosses the date line at precisely midnight, going westward, one skips an entire day; while going eastward, one repeats the entire day.