The International Date Line is an imaginary set by the humans so that it can be used as a starting point for counting the time. This line is actually a meridian located in the Pacific Ocean, and it is on the total opposite side of the Prime Meridian, the Greenwich Meridian.
This line makes something very interesting when crossed. If the line is crossed from east to west, than the travelers are immediately gaining a day, or rather one hour that moves them into another time zone that is part of another day. The reason for this is that west of the line the day starts, while east of the line, the day ends.
If someone moves in the opposite direction, from west to east, than a day will be gained immediately after the crossing of the line. In this manner, by traveling through this line, as well as the other time zones, someone can gain multiple hours, or lose multiple hours.
