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 include Kiribati’s 1995 shift to bring its easternmost islands onto the same calendar day as the rest of the country, and Samoa’s 2011 change to align its business week with Australia and New Zealand. The 1884 International Meridian Conference established the prime meridian at Greenwich and helped standardize time zones, but did not formally establish a single date line; its location emerged through practice and bilateral agreements.

Practical implications concern navigation, aviation, telecommunications and legal matters such as contracts and deadlines. The line’s irregular path avoids dividing inhabited territories and takes into account local administrative needs, which can produce counterintuitive date and weekday relationships between nearby locations. Discussions about the datumslinie also touch on cultural and economic considerations when states choose to alter their position relative to it.