The Mathematics of the International Date Line

The International Date Line (IDL) is where the calendar day changes. It roughly follows the 180° meridian but zigzags to avoid cutting through countries. Crossing eastward subtracts a day; westward adds a day. The IDL exists because of the spherical geometry of Earth and the need for a consistent global time system. Without it, a traveller circumnavigating west would gain a day (as in Magellan’s crew, who missed a day and thought it was Wednesday when it was Thursday). The IDL is a mathematical discontinuity necessary for a continuous time coordinate.