Datumslinie
Datumslinie (literally "date line") is a term used to describe the imaginary demarcation on Earth's surface where the calendar date changes by one day. In practice it corresponds to the International Date Line, which mostly follows the 180th meridian but deviates to accommodate political boundaries, island groups and practical timekeeping needs. Crossing the line from east to west advances the calendar by one day; crossing from west to east subtracts one day.
The datumslinie is not fixed by international treaty and has been shaped by national decisions. Notable adjustments
Practical implications concern navigation, aviation, telecommunications and legal matters such as contracts and deadlines. The line’s