You may be aware that to keep time while traveling, you must change your watch by one hour every time you cross a time zone boundary. When traveling west, you subtract one hour when you enter a new time zone. Conversely, for every time zone boundary you cross going east, you add one hour. Because Earth is round, if you continue to travel in one direction, you will eventually return to your starting point, and you would have set your watch forward or backward a full day.

In order to limit time zone confusion for travelers, the International Date Line was established. The line runs along 180 degrees longitude, detouring around countries as needed to keep them all on the same calendar day. Locations to the west of the International Date Line are one calendar day ahead of locations to the east of the line.

  !   Click the image to see a simulated trip across the International Date Line. Use it as a visual check as you calculate the time and date of the ending of the trip described below.