International Date Line

What it is, and why it has no official definition

Travel around the world eastward and you set your watch ahead an hour every 15°, so by the time you get home you’ve gained a whole day; go westward and you lose one. Somewhere the calendar has to make up for that jump, and that place is the date line: west of it (Asia and Oceania) it’s one day later than east of it (the Americas).

It roughly follows the 180° meridian, opposite Greenwich, because it’s almost all ocean out there. But no treaty or international law sets it: it simply results from the time each country chooses for its territory. That’s why it has no official course, and when a country switches time zones, the line moves with it.

It’s drawn here with Natural Earth’s geometry (public domain), which already includes the changes made by Kiribati and Samoa. Out at sea it’s only an approximation: what really counts is the time each island keeps.

Where it zigzags

The line bends so it doesn’t split countries in two. In the north it keeps all of Chukotka (Russia) on the Asian side and the Aleutian Islands (Alaska, U.S.) on the American side. In the central Pacific it makes a huge detour east to put all of Kiribati on the same day, straying up to 30° from the 180° meridian, about 2,044 mi (3,290 km). Farther south it leaves Samoa, Tokelau, Tonga, Fiji and New Zealand to the west, and American Samoa, Niue and the Cook Islands to the east.

Here are 17 places on either side, with their distance to the line and their offset from UTC. West of the line, clocks run ahead of UTC; east of it, behind:

In the Bering Strait the line runs between the two Diomede Islands, Big Diomede (Russia) and Little Diomede (U.S.), about 2.5 mi (4.0 km) apart. Their clocks are 21 hours apart in winter and 20 in summer, when Alaska springs forward. There, Natural Earth simplifies the line along the 169° W meridian; the actual maritime border runs slightly east, at 168° 58′ 37″ W.

Tonga and American Samoa, a few hundred miles apart, show the same time on the clock but are 24 hours apart: they’re always a day apart. The same goes for Kiritimati (Kiribati) and Honolulu (Hawaii). And on New Zealand’s Chatham Islands, the time is UTC+12:45 in winter, one of the few time zones with a 45-minute offset.

Kiribati in 1995, Samoa and Tokelau in 2011: when the line moved

In 1892 Samoa switched from the Asian side to the American side to make trade with the United States easier: it repeated Monday, July 4, U.S. Independence Day.

Kiribati, independent since 1979, was split in two: the Gilbert Islands, with the capital, were a day ahead of the Phoenix and Line Islands. To unite the country it skipped Saturday, December 31, 1994, going straight from Friday the 30th to Sunday, January 1, 1995. Since then the Line Islands have had the earliest time on Earth, UTC+14, and Caroline Island, one of the first pieces of land to reach the year 2000, was renamed Millennium Island.

In 2011 Samoa moved back to the Asian side to share the same day as Australia and New Zealand, its main partners: it skipped Friday, December 30, 2011, and went from UTC−11 to UTC+13. Tokelau, a New Zealand territory, made the same switch. American Samoa, a U.S. territory, stayed on the other side.

Three dates at once

Between 10:00 and 12:00 UTC every day, three calendar dates exist at the same time: on Kiritimati (UTC+14) it’s already tomorrow, in most of the world it’s today, and on Baker and Howland Islands, uninhabited and on UTC−12, it’s still yesterday. Next to the clocks above, you can see whether there are two or three dates right now.

Kiritimati is the first inhabited place to ring in the New Year, and American Samoa and Niue (UTC−11) are among the last.

Magellan’s missing day

The first trip around the world already ran into the line. In July 1522, on reaching Cape Verde, the survivors of Magellan and Elcano’s expedition had Wednesday, July 9, in their log, but locals told them it was Thursday, July 10. They hadn’t missed a single day in their records: by sailing around the world westward, they had “lost” one.

It’s the same twist as in Jules Verne’s Around the World in Eighty Days: Phileas Fogg travels east, gains a day without noticing and makes it back in time to win his bet.