dátumvonal

A dátumvonal is an imaginary line on the surface of the Earth used to mark where calendar dates change by one day. In common usage it refers to the International Date Line, which roughly follows the 180th meridian in the central Pacific, opposite the Prime Meridian. The line is not a fixed political boundary; its course is adjusted to avoid splitting landmasses and national territories between consecutive dates.

How it works: when crossing the dátumvonal from west to east, the date is set back by one day; crossing from east to west adds one day. Time zones near the line are irregular, and national governments determine local time offsets, sometimes leading to abrupt date changes in travel itineraries or software systems. The line’s exact path is designed to maintain calendar continuity for inhabited regions, so it zigzags around islands and states.

History and practice: the concept emerged with the adoption of standardized time zones in the 19th century and the International Meridian Conference of 1884, which helped establish Greenwich as the prime meridian. Since then, the dateline has been adjusted several times to reflect political and economic considerations. Notable changes include Kiribati’s move in the 1990s to put all its eastern islands on a single date by shifting the line, and Samoa’s 2011 adjustment to align with its major partners, effectively moving its territory to the other side of the line.

Applications: the dátumvonal affects navigation, aviation, shipping, and daily planning, where the calendar date must be kept consistent across longitudes. In geography and education, it serves as a practical reference for understanding how date and time relate to longitude and time zones.