The International Date Line

Where the day starts

The International Date Line is an imaginary line from the North Pole to the South Pole that separates one calendar day from the next. For most of its length it follows the 180th meridian, halfway round the world from Greenwich.^[1]

It exists because clocks everywhere are set so that noon comes roughly when the sun is highest. A traveller going all the way round the world, changing their watch at each time zone, would come home a day out from everyone who stayed put. So travellers crossing the line eastward set their calendars back a day, and those going west set them a day ahead.^[1]

The line bends. It swings east through the Bering Strait so as not to split Siberia, west to keep Alaska’s Aleutian Islands with Alaska, and east again south of the Equator so that some island groups share New Zealand’s day.^[1]

The lost day

The first people to notice were the survivors of Magellan’s voyage round the world. When they reached the Cape Verde Islands in July 1522 and sent men ashore, they were told it was Thursday. By the chronicler Antonio Pigafetta’s careful daily log it was Wednesday. Sailing westward with the sun for three years had cost them a day.^[5]

For centuries the Pacific had two day counts. The Portuguese, Dutch and English reached Asia sailing east and kept an Asian count, while Spain settled the Philippines from Mexico and kept the American one. Manila was a day behind Macao, although the two were close neighbours.^[6]

Countries that changed sides

In 1844, to trade more easily with its Asian neighbours, the Philippines switched to the Asian count: its governor-general proclaimed that Monday 30 December 1844 would be followed by Wednesday 1 January 1845.^[6]

Alaska went the other way. It had kept the Russian settlers’ Asian day and their Julian calendar, and after the United States bought it in 1867, Friday 6 October was followed by Friday 18 October.^[7]

In 1892 an American trading house persuaded King Malietoa Laupepa of Samoa to adopt the American day. As a compliment to the Americans, the change was made by celebrating Monday 4 July twice.^[7]

A line nobody drew

When the International Meridian Conference of 1884 chose Greenwich as the prime meridian, delegates noted how conveniently the 180° meridian opposite it passed mostly over water. But the conference never said where the date line should go where it met land or island groups, and no treaty, law or international agreement has defined it since.^[2] So “International” is really a misnomer.^[2]

That leaves countries free to move it. Kiribati, independent since 1979, stretched across the line, so its western islands were always a day ahead of its eastern ones, leaving only four days a week when both halves were at work. From 1 January 1995 President Teburoro Tito had the line follow Kiribati’s eastern edge, so on its eastern islands Friday 30 December 1994 was followed by Sunday 1 January 1995.^[3] That gave the line, which had stayed close to the 180° meridian for most of the 20th century, a very noticeable bulge to the east.^[3]

Samoa and Tokelau crossed back at the end of 2011, to be closer in time to New Zealand and Australia. When midnight struck at the end of Thursday 29 December, they jumped straight to Saturday 31 December, 119 years after Samoa had moved the other way.^[4][3] Samoa’s prime minister said Samoans would now have five working days of continuous contact with New Zealand and Australia.^[4]