The International Date Line Explained
The International Date Line is an imaginary line in the Pacific, roughly along 180 degrees longitude, where the calendar date changes. Cross it heading east and you go back a day; cross it heading west and you jump forward one. It also zigzags quite a bit, so that islands and countries can stay on a single date.
Why the date has to change somewhere
Midnight sweeps round the planet one time zone at a time, so somewhere the date has to roll over. Without one agreed place for that to happen, anyone who travelled all the way round the world would come home a day out of step with everybody else. The date line is that agreed place.
Crossing it
Fly west, say from the US to Japan, and you move forward a day, which is how you can leave on Monday and land on Wednesday. Fly east, from Japan to the US, and you go back a day, so you can land on the same date you left, or what feels like an earlier one.
You don't actually gain or lose any time, of course; only the date label changes. The practical lesson is to write the date and not just the time whenever people on both sides of the line are involved.
Why it bends
It bends around countries and island groups so each one keeps a single date. Imagine neighbouring islands sitting on different days; trade and everyday life would be a constant headache.
Places that moved across it
Samoa made the most famous jump. In 2011 it moved to the west side of the line and skipped 30 December that year entirely, to line its working week up with Australia and New Zealand, its main trading partners.
Kiribati had the opposite problem: its islands sat on both sides of the line, so parts of the country were a whole day apart. It rezoned in 1995, and its Line Islands now use UTC+14, the earliest time zone in the world. At the far end, two uninhabited US islands, Baker and Howland, sit on UTC-12, the latest zone.
Where the new day and the new year begin
In the places furthest ahead, around UTC+14 and UTC+13, which means parts of Kiribati, Samoa and Tonga. New Zealand follows soon after, then Australia, and the rest of the world comes along westward. Our New Year countdown page shows when midnight reaches each place.
What it means for your calendar
If you work with people in New Zealand, Fiji or Australia from the Americas, your Monday is their Tuesday, so write the date and the zone and check it with the Time Zone Converter. And if you're wondering whether the line is simply 180 degrees longitude: roughly, yes, except where it bends around land. The background is in how time zones began.