If you travel west around the world, setting your watch back an hour every fifteen degrees, you'll have set it back a full 24 hours by the time you get home. Your watch will show the right time, but your calendar will be a day behind. Somewhere, the date has to change. The International Date Line is the agreed place where it does.
Discovering the missing day
The problem was noticed long before time zones existed. When the survivors of Magellan's expedition reached the Cape Verde Islands in 1522 after sailing westward around the globe, their carefully kept logbook was a day behind the date on land. They hadn't miscounted; they'd experienced one fewer sunrise than people who stayed put.
Where the line is
The date line roughly follows the 180° meridian, on the opposite side of the world from Greenwich. That's a convenient choice because it runs almost entirely through open ocean. Crossing it westbound (from the Americas towards Asia), you skip forward a day; crossing eastbound, you repeat one.
It isn't set by any international treaty. It's a convention that follows the time zone choices of the countries near it, which is why it bends. It swings east around Russia's Chukotka region so all of Russia shares a date, west around the Aleutian Islands to keep Alaska together, and far east around Kiribati.
Countries that moved across it
Because the line follows national decisions, it moves when those decisions change.
- Samoa, 2011. Samoa had been east of the line, on the same day as the United States, since the late 1800s. By the 2000s most of its trade was with Australia and New Zealand, which were a day ahead. At the end of Thursday 29 December 2011, Samoa jumped straight to Saturday 31 December. Friday 30 December never happened there. Neighbouring Tokelau made the same switch.
- Kiribati, 1995. Kiribati's islands stretch across both sides of 180°, so for years the country's eastern and western parts were on different days and only shared four working days a week. From 1 January 1995 it moved its eastern islands to the Asian side of the line, pushing the line into a large eastward bulge.
- The Philippines, 1844. Colonised from the Americas, the Philippines originally kept the American date. It switched to match its Asian neighbours by skipping 31 December 1844.
Why some places are 26 hours apart
You might expect local times to span 24 hours. They actually span 26. Kiribati's Line Islands, including Kiritimati, run on UTC+14, the earliest time zone on Earth. The uninhabited U.S. islands of Baker and Howland sit in UTC−12. At noon on Tuesday in Kiritimati it is 10:00 on Monday on Baker Island, even though both lie in the central Pacific, on the same side of the 180° meridian.
That gap is why for about two hours of every day, three different calendar dates are in use somewhere on Earth at the same moment.
"Anywhere on Earth"
The UTC−12 zone has an unexpected job: deadlines. Academic conferences and grant programmes often set deadlines as "23:59 AoE," Anywhere on Earth. Because UTC−12 is the last place on the planet where a given date ends, a deadline in AoE has not passed anywhere until it has passed everywhere.
The date line and the new year
Kiritimati and the Line Islands are the first inhabited places to reach midnight on 1 January, followed by New Zealand's Chatham Islands and then Auckland. American Samoa and Niue, east of the line, are among the last, nearly a full day later. Samoa itself, after its 2011 move, went from being one of the last countries to welcome the new year to one of the first.
What it means for travellers
On a flight from Los Angeles to Sydney you lose a calendar day; coming back you regain it and can land "before" you left. Hotel bookings, visa dates and connecting flights should all be checked against the local date on arrival, not counted from the departure date. The flight times guide walks through a worked example.