Final answer:
Crossing the International Date Line from east to west adds one day to the calendar. This ensures that travelers who circumnavigate the globe have the correct date despite time zone changes.
Step-by-step explanation:
Traveling from east to west over the International Date Line advances the calendar by one day. The Date Line serves as an imaginary marker that helps synchronize the calendar dates for global timekeeping. It's set by international agreement to run approximately along the 180° meridian of longitude, primarily in the middle of the Pacific Ocean.
When crossing this line from east to west, you effectively 'gain' a day on the calendar due to the way time zones are structured around the world. This adjustment ensures that after traveling around the world, and moving your watch ahead by an hour in each new time zone, you'll end up with the correct date once you complete your journey. The International Date Line is essential to maintain a consistent and rational system of timekeeping worldwide.