Final answer:
The International Date Line approximately follows the 180° meridian and alters local dates when crossed. It circumvents some regions for temporal uniformity within those areas, adjusting the date by one day depending on the direction of travel.
Step-by-step explanation:
The International Date Line is an imaginary boundary from which the calendar date changes. It is set by international agreement to roughly follow the 180° meridian of longitude, directly opposite the Prime Meridian. The line serves as a 'date separator' that adds or subtracts a day when passed. It mostly traverses the Pacific Ocean, but it circumvents certain areas, such as island groups and Alaska, to maintain consistent timekeeping within these regions. When one travels across the International Date Line, the date advances by one day if going west and decreases by one day when heading east to manage the discontinuity that results from the Earth's rotation and the division of time zones.