The International Date Line sits near the middle of the Pacific Ocean and shifts the calendar by a day.

The longer version

Somewhere on the globe a new calendar day has to begin, and the chosen place is the 180th meridian, which runs mostly through open ocean. Crossing it changes the date while leaving the clock time almost untouched, so a short hop can add or remove a day. Its position in the Pacific was chosen precisely because few people live there.

The line is not straight. It bends around island groups and past territories so that neighbours sharing an economy keep the same calendar day. Without such detours, communities a few miles apart would be living in different days, which would make trade and schooling unworkable. Ships and aircraft reset their date at the crossing and log it carefully.

Where this entry sits

These facts wander through clocks, calendars, time zones, and the long human effort to measure passing hours. You will find leap years, odd month lengths, ancient sundials, and the reasons modern timekeeping works the way it does. The full list sits on the Time and Calendars page, where every entry is listed in order.