International Date Line (IDL)

An imaginary longitudinal line located at approximately 180 degrees longitude serving as the demarcation where the calendar date changes by one day when crossed.

The International Date Line (IDL) is a conceptual boundary on the Earth’s surface, primarily situated along the 180th meridian in the mid-Pacific Ocean. It is not a straight line but zigzags to accommodate political and territorial boundaries, ensuring that countries and territories remain within the same calendar day. The IDL plays a crucial role in the global timekeeping system, as it marks the point where the date changes by one day. Travelling eastward across the line results in moving back one calendar day, while travelling westward adds one day to the calendar.

The line is not defined by international law but is recognised by convention and is integral to the Coordinated Universal Time (UTC) system. It ensures consistency in global time zones and helps maintain a standardised framework for navigation, communication, and international agreements. For mariners, the IDL is particularly significant when planning voyages across the Pacific Ocean, as it affects logbook entries, scheduling, and coordination with ports of call. By understanding the IDL, sailors can accurately adjust their ship’s clocks and calendars, ensuring seamless integration with global timekeeping practices.