The International Date Line
In the middle of the Pacific Ocean runs an invisible line where two consecutive days meet. Cross it heading west and you jump a day forward. Head east and the calendar goes back one day. This is the International Date Line — and it is anything but straight.
The principle: the counterpart of Greenwich
The time zone system takes the 0° meridian through Greenwich as its starting point — read more on the page UTC, GMT and the prime meridian. Travel 180° eastward or westward from there and you reach the exact opposite side of the globe: the 180° meridian. That meridian runs almost entirely through the Pacific Ocean, far from most inhabited areas.
The time zone system has 24 zones in theory, each 15° wide. West of the prime meridian the clock lags behind UTC; east of it the clock runs ahead. At the 180° meridian the difference is at its maximum: UTC+12 on the western side and UTC−12 on the eastern side. Those two zones are exactly 24 hours out of phase. That is precisely where the date jumps.
The Date Line is not a formal treaty but an international convention determined country by country. No international treaty fixes its route — each country decides which side it wants to be on (source: IANA Time Zone Database).
Why does the line zigzag?
A perfectly straight line along the 180° meridian would cut through inhabited areas and split island groups. Nobody wants that: it would mean one half of an island has Monday while the other has Tuesday. So the Date Line bends around populated islands in a series of curves:
Kiribati (bend to the east) — The island republic of Kiribati stretches both east and west of the 180° meridian. In 1995 Kiribati moved its time zone so that all its islands would share the same date, even though some lie far east of 180°. As a result the Date Line curves far eastward around Kiribati. The consequence: Kiribati is the first country on Earth to begin a new day. The island of Caroline (now Millennium Island) was the first place in the world to enter the new millennium on 1 January 2000.
Samoa (bend to the west) — Until 2011 Samoa lay on the eastern side of the Date Line, at UTC−11. The country traded with Australia and New Zealand but was a day behind its main trading partners. On 29 December 2011 Samoa crossed the Date Line to UTC+13 — a Friday that officially never existed for the island's population. Now Samoa is a day ahead of its neighbour American Samoa (which stayed on the other side).
Aleutian Islands (bend to the east) — The westernmost Aleutian Islands of Alaska lie geographically further west than Hawaii, but belong to the US and therefore use Alaska Time (UTC−9 to UTC−10), so islanders share the same date as the rest of the US. The Date Line bends eastward to loop around these islands.
The zigzag line in view
What happens when you cross the line
The Date Line does not directly affect the clock — the time changes with the normal time zone transition. But the date jumps:
Crossing westward (from the eastern to the western side of the line): your calendar jumps one day forward. You 'lose' a day: that day simply does not exist on your travel route. Flights heading to Australia from America make this jump.
Crossing eastward (from the western to the eastern side): your calendar jumps one day back. You experience the same date twice. Passengers on cruise ships sailing west to east across the Pacific live through two afternoons on that day.
Airlines always show the arrival date explicitly on tickets to avoid confusion. Someone flying from Amsterdam via Tokyo to Los Angeles 'gains' a day back after crossing the Date Line.
Worked example — flight from Sydney to Los Angeles
Departure: Sunday 10 May, 09:00 local time in Sydney (UTC+10).
Flight duration: approximately 14 hours.
Arrival local LA time (UTC−7): Sunday 10 May, 06:00.
The aircraft has crossed the Date Line eastward and the date has gone back one day. The journey takes 14 hours, but it appears as if you arrive before you left.
The day begins in Kiribati
Because of its unique position east of the Date Line yet with UTC+14, Kiribati is the first country on Earth where every new day begins. Millennium Island — formerly Caroline Island — is the first inhabited place in the world where the clock strikes midnight. Tourists travel there to be among the first to celebrate the new year.
The Pacific region is geographically and culturally extraordinarily diverse — the islands of Oceania together cover the largest ocean area of any continent or region. The surrounding oceans are, partly because of the Date Line, the most complex time zone region on Earth.
Sources
- IANA Time Zone Database — tz.iana.org (time zone names and offsets for the Pacific region)
- Royal Observatory Greenwich — explanation of the Date Line and its history
- CIA World Factbook — time zone information for Kiribati, Samoa, Aleutian Islands
- Samoa Ministry of Commerce — announcement of the Date Line switch, December 2011