1. The Width of a Day
2. Threads

a thread from chapter II

How the date line works

Nobody owns the line between today and tomorrow. It is a habit, drawn by mapmakers, bent by governments, and in one place it runs between two islands you can see across.

1522The day that went missing

When the last ship of Magellan's fleet reached the Cape Verde islands in July 1522, its chronicler, Antonio Pigafetta, had kept a record of every day of the voyage. By his count it was Wednesday. On shore it was Thursday.¹

Nobody had miscounted. Sailing west around the world, the ship had followed the sun, and each of its days had been a few minutes longer than a day at home. Add up a whole circuit and you lose one full day. Sail east and you gain one.

So if travelers are going to agree with the people at home, they have to change the date somewhere. That somewhere is the International Date Line.

no treatyA line nobody signed

There is no international agreement that fixes the date line. It is a cartographic convention, not a law.² When the International Meridian Conference met in 1884 and chose Greenwich as the prime meridian, it explicitly declined to set up time zones at all.²

Roughly, the line follows the 180° meridian, halfway around the world from Greenwich, because that runs mostly through open Pacific. But wherever it would cut through a country, it bends.²

Bering StraitThe bend at the top of the world

In the far north the line swings east around Chukotka, so all of Russia keeps Asia's date, and then passes straight through the Bering Strait between the two Diomede Islands.² South of the strait it swings back west, so that the Aleutian Islands stay on Alaska's date.²

That is why Big Diomede, in Russia, keeps Chukotka time (UTC+12) and Little Diomede, in Alaska, keeps Alaska time (UTC−9, or UTC−8 in summer).³ The difference is 21 hours in winter and 20 in summer. Only 2.4 miles (3.8 km) of water separates them.

Why not 24? Because the zones are not exactly opposite. Chukotka is 12 hours ahead of UTC and Alaska is 9 behind: 12 + 9 = 21.

1867When Alaska moved to America's calendar

Under Russia, Alaska had kept Russia's date. When it passed to the United States on October 18, 1867, the date moved back to match America's side of the world, and the change from Russia's Julian calendar to the Gregorian one happened at the same moment. That day is still marked as Alaska Day.²

1995 and 2011Countries that jumped across

Kiribati is spread across both sides of the 180° meridian. Until 1994, its eastern and western islands were a whole day apart. The government deleted Saturday, December 31, 1994, and the line has bent around the entire country ever since.² Its Line Islands now keep UTC+14, so they are among the first places on Earth to reach each new day, and each new year.²

At the end of 2011, Samoa jumped the other way. Thursday, December 29 was followed directly by Saturday, December 31, putting Samoa on the same days as Australia and New Zealand, its main trading partners. Tokelau jumped with it.⁴

right nowHow many dates are there?

Inhabited time zones run from UTC−11 to UTC+14, a span of 25 hours. So for most of each day, people on Earth are living in two different dates, and for about an hour a day, three. The main story counts them live.