If you cross the you will either pass into the next day or the previous day, depending on whether you sre moving east or west.

Internation Date Line. The International Date Line is an imaginary line that crosses the surface of the Earth and goes from the North Pole to the South Pole, and it signifies the change of one calendar day to the next. It crosses the middle of the Pacific Ocean, over the 180° line of longitude expect for some territories and island groups.