What is the International Date Line?

The date line runs roughly along the 180th meridian, opposite the Prime Meridian, and marks where one calendar day ends and the next begins. Traveling west across it advances the date by one day, so a Tuesday flight from Los Angeles can land in Tokyo on Wednesday. Traveling east subtracts a day, which produces the disorienting experience of landing before you took off in local time. The line is a convention rather than a treaty-defined boundary, and countries set their own dates, which is why it bends.

Those bends are politically driven and occasionally dramatic. The line detours east around Kiribati so that the entire nation shares one date, a change made in 1995 that placed the country's easternmost islands ahead of everyone else and made Kiribati the first place to enter each new year. It swings west around the Aleutians to keep Alaska unified and around Russia's far east. Samoa jumped the line entirely in 2011, skipping December 30 outright, to align its business week with Australia and New Zealand rather than the United States. The line's practical importance is mostly administrative now, though it still creates the peculiarity that two islands in the Diomede group, less than three miles apart, sit almost a full day different from each other.