Crossing the International Date Line westward moves you forward one calendar day.

The longer version

Heading west across the line means you move from a place where the date is earlier to one where it is later, so you add a day: Tuesday becomes Wednesday in an instant. Heading east does the reverse and you repeat a date. The clock face barely changes; only the calendar does.

Travellers often find the arithmetic counterintuitive, because they expect direction and gain to line up neatly. The confusion grows on long flights that cross many zones as well as the line, since the clock and the calendar move independently. Booking systems account for it, but printed itineraries still surprise people who count the hours on their fingers.

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.