The International Date Line
Cross it westward and tomorrow arrives at once; cross it eastward and you live the same day twice. Its path is a set of national decisions, not a line of longitude.
Somewhere on Earth the calendar has to turn over, and that somewhere is the International Date Line. It runs roughly along the 180th meridian through the Pacific, on the far side of the world from Greenwich, so that the day begins there and spreads westward around the globe. The world clock makes the date change visible: add Auckland and Honolulu and they differ by 22 or 23 hours, and for most of each day are on different dates.
What it does
Travel west across the line and the date moves forward by one day: a flight leaving Los Angeles on Tuesday evening lands in Sydney on Thursday morning, though it was in the air for fifteen hours. Travel east and the date moves back, so a flight from Tokyo can land in Los Angeles earlier, by the clock and the calendar, than it left. Nothing happens to the time of day at the line itself; two points on either side of it can be an hour apart in clock time while being a day apart in date.
Why it is not straight
The line has no legal definition and no treaty behind it; it is simply the consequence of the time zones countries have chosen for themselves. It bends east around Russia’s Chukotka so that the whole of Russia is on the same side as Moscow, and bends west around the Aleutian Islands so that all of Alaska shares a date with the rest of the United States. Alaska itself changed sides when it was bought from Russia in 1867, moving from the Asian to the American date and from the Julian to the Gregorian calendar at the same time, with the result that Friday 6 October was followed by Friday 18 October.
Kiribati, 1995
The largest bend is in the central Pacific. Kiribati’s islands straddled the 180th meridian, so the country’s eastern and western halves were on different dates and shared only four working days a week. In 1995 the government moved the eastern islands to the western side of the line, putting the whole country on one date. The Line Islands took the offset UTC+14, the largest in the world, so that Kiritimati (Christmas Island) is now the first inhabited place to reach each new day, a full 26 hours before the last. The line now bulges far to the east of 180° to go around them.
Samoa and Tokelau, 2011
Samoa had been on the eastern side of the line since 1892, when it moved to align with trade from the United States. By 2011 its main partners were Australia and New Zealand, a day ahead, and the country lost two business days a week to the mismatch. At midnight on 29 December 2011 Samoa and Tokelau moved across the line; Friday 30 December did not occur, and the next day was Saturday 31 December. Samoa went from being among the last places to see each date to among the first. American Samoa, a few dozen kilometres away, stayed where it was and is now a day behind its neighbour.
The 26-hour world
With UTC+14 at one extreme and UTC−12 at the other, the world’s clocks span 26 hours. At any instant there are up to three calendar dates in use somewhere on Earth: for two hours each day, the Line Islands are on one date, most of the world on the previous one, and Baker and Howland Islands, uninhabited US territories at UTC−12, on the one before that. The last inhabited places to end each date are American Samoa and Niue at UTC−11.
Who sees the new year first
Kiritimati sees it first, at 10:00 UTC on 31 December. The Chatham Islands follow at 10:15 UTC, on their summer offset of +13:45, then New Zealand, Samoa and Tonga at 11:00 UTC. Sydney reaches it at 13:00 UTC, Tokyo at 15:00, Delhi at 18:30, London at midnight UTC, New York at 05:00 UTC on 1 January, Los Angeles at 08:00, Honolulu at 10:00, and American Samoa last among inhabited places at 11:00 UTC, twenty-five hours after Kiritimati. The New Year countdown lists the order for any year, and the day and night map shows where midnight currently is.
Why it matters for software
The date line is why a date without a zone is ambiguous by up to two days, not one. A deadline of “31 December” spans from 10:00 UTC on 30 December, when it begins in Kiritimati, to 11:59 UTC on 1 January, when it ends in American Samoa. Any system that stamps a plain date on an event and later compares it with a timestamp needs to know which zone the date was in, which is one of the reasons the guide to storing dates keeps dates and instants apart. The database that records all these offsets is described in the time zone database guide.
A line that keeps moving
Because the line follows national choices, it moves whenever a Pacific nation changes its offset, and several have discussed doing so. Nothing on a map fixes it. The reliable description of where the date changes is the time zone database itself: for any two places, the dates differ exactly when their offsets put them on opposite sides of midnight, and the world clock is the simplest way to see that for any pair.
Sources
Frequently asked questions
Where exactly is the International Date Line?
Roughly along the 180th meridian in the Pacific, bending around Russia, Alaska, Kiribati, Samoa and Tonga according to the time zones those countries have chosen. It has no legal definition of its own.
Which place sees the new year first?
Kiritimati in Kiribati, at UTC+14, followed by the Chatham Islands and then New Zealand, Samoa and Tonga. American Samoa and Niue, at UTC−11, are the last inhabited places.
Why does the date line go around Kiribati?
In 1995 Kiribati moved its eastern islands to the western side of the line so the whole country shared one date. The Line Islands took the offset UTC+14 and the line now bulges east to go around them.
What happened when Samoa crossed the date line in 2011?
Friday 30 December 2011 was skipped. Samoa and Tokelau went from being among the last places to reach each date to among the first, to align their week with Australia and New Zealand.