The International Date Line is where the calendar day changes. It runs roughly along the 180° meridian, through the middle of the Pacific, halfway round the world from Greenwich. Cross it heading west and you jump forward a day; cross it heading east and you go back one. It isn't a straight line, and it isn't set by any treaty: it follows the choices of the countries near it.
Why there has to be one
Travel east and every time zone you cross puts your clock an hour ahead. Keep going all the way round and you'd be a whole day ahead of the people you left. Somewhere, the day has to be set back again. The survivors of Magellan's expedition discovered the problem in 1522: after sailing west round the world, their carefully kept log was a day behind the calendar in Cape Verde.
Why it zigzags
The line bends to keep countries and island groups on one date. It swings east around Kiribati, north-east of Fiji, and west around Alaska's Aleutian Islands so that they share a date with the rest of Alaska. Moves have happened within living memory:
- Kiribati, 1995. Spread across both sides of the line, the country moved its eastern islands to the Asian side, so the whole nation shares one date. Its Line Islands are now at UTC+14, the earliest time on Earth.
- Samoa, 2011. Samoa skipped Friday, December 30 entirely, jumping from Thursday to Saturday, to move to the west side of the line and line up its working week with Australia and New Zealand, its main trading partners.
The earliest and latest places
Because of these bends, the world's clocks span 26 hours rather than 24. Kiribati's Line Islands at UTC+14 see each new day first; uninhabited Baker and Howland Islands, at UTC−12, are the last. For two hours each day, three different dates are in use somewhere on Earth.
Flying across it
Fly from Auckland to Honolulu and, by the clock, you land before you took off: in January, Auckland is +23h ahead of Honolulu, so an evening departure arrives on the morning of the same date. Going the other way, from Los Angeles to Sydney, an overnight flight seems to swallow a whole day. Your body, of course, doesn't care about dates. It cares about how many hours the clocks moved, and it takes the shorter way round; the jet lag calculator shows the shift for any trip.
Keeping track
The world clock shows the day as well as the time for every city you add, and the time zone converter flags when a time falls on the next or previous day. For more on why zones are drawn where they are, see time zones explained. And in Flight, you can cross the date line at your desk on a focus flight across the Pacific.