Taylor Swift flies from Japan to Las Vegas and gains several hours. Hong Kong’s stock market closes while London’s opens. On a politician’s orders, a clock on a remote Pacific island strikes midnight 24 hours early.
None of these times are empirical scientific facts. People have just agreed to observe time zones, a concept promoted by railroad companies in the 19th century.
But time zones have physical dimensions. So where exactly do the days begin and end? The short answer is that Monday becomes Tuesday at the International Date Line, a border that runs through the Pacific Ocean.
The longer answer is that there are no international rules determining the location of the date line, and its exact coordinates depend on the varying whims of governments. Maps that try to show this are never quite right, and the line itself technically doesn’t exist.
Confused? Here’s an introduction.
People have been talking about this for centuries.
The idea of establishing a line where days begin and end has been around since the 13th century. But while the equator is a logical dividing line between the Northern and Southern Hemispheres, there is no obvious place to separate the Eastern and Western Hemispheres.
For a long time, mapmakers chose their own east-west dividing lines, called meridians, a word derived from the Latin for “noon.” In the absence of an international standard for when days began or ended, sailors on long sailing voyages had to decide for themselves how to account for the time lost or gained.
A 16th-century account of a voyage by English explorer and pirate Francis Drake described a ship arriving on a Sunday. But according to ‘the ordinary reckoning of those who had remained at home in one place or country’, it was already Monday.
The location of the date line has never been determined.
In 1884, 25 countries passed a resolution calling for a ‘prime’ meridian fixing zero degrees longitude in Greenwich – a town on the outskirts of London with a royal observatory – to create an international reference point for mapmakers, timekeepers and train planners. They also decided to establish a ‘universal day’.
But it took decades for many countries to accept the Prime Meridian and formalize the time zones associated with Greenwich, according to Ian R. Bartky’s 2007 book “One Time Fits All.” And the physical location of that universal day – the International Date Line – was never formally recorded.
In 1921, the British Admiralty, which administered Britain’s maritime affairs, said that no date line had “ever been definitively fixed, either by any particular power or by international agreement.” That still holds true over a century later.
“While the Prime Meridian feels sacred, the International Date Line is not a meridian; it’s pretty arbitrary,” Tim Montenyohl, a cartographer who has mapped the date line, wrote in 2018.
Some countries have moved the date line.
Because the concept of the international date line is not enforced by any international treaty, countries and territories in the Pacific are essentially free to decide which side they want to be on. Some have switched sides for political or commercial reasons.
Spain initially placed the Philippines, its 16th century colony, on the eastern side of the time change. That essentially forced the date line to swing west from the 180th meridian. But in 1844, the Philippines moved the line by declaring that the day of December 31 of that year would “lapse as if it were really past.”
Some Pacific island nations have moved the date line unilaterally to simplify local timekeeping or to boost trade relations within the Asia-Pacific region.
In the 1990s, Kiribati moved the line eastward along the 180-degree meridian to include the easternmost islands. In 2011, Samoa – which, at the insistence of American traders, had jumped the same meridian in 1892 by observing the same Monday twice – hit back by shortening a Friday.
Emma Veve, an economist at the Asian Development Bank who has worked in the Pacific islands, said Samoa’s move made commercial sense because it put the country on the same working day as New Zealand. While the news media made a fuss, they said, people went on with their lives.
The date line still challenges map makers.
For mapmakers – and reporters – the international date line can be difficult to determine.
Cartographers typically map it by consulting other maps, including a time zone published by the Central Intelligence Agency. But creating a more detailed version is complicated, Mr. Montenyohl said. This is partly because countries change time zones; digital maps tend to reflect the shortcomings of the pre-digital maps on which they were based; and a country’s territory extends up to 200 nautical miles from its land borders.
“It breaks your brain very quickly if you get too deep into the weeds,” he said.
Here’s a nice example.
In 2020, journalist Johnny Harris noticed a discrepancy between two views of the date line around some of the Cook Islands, in the South Pacific.
“Google says these islands are on the Tuesday side, which means the one-day-ahead side, while PacIOOS says these are on the Monday side, meaning the one-day-back side,” Mr. Harris said in a YouTube video. referring to the Pacific Islands Ocean Observing System, a nonprofit organization based in Hawaii.
So which version is right? It’s still not completely clear.
A spokeswoman for the Cook Islands government did not answer the question. A Google spokesperson said only that the company’s dateline maps have been updated since 2020. And a data systems engineer at PacIOOS said the group’s version was not a gold standard.
“We are certainly not experts or authorities on the date line,” said the engineer, John Maurer. He added that PacIOOS used the same version as Wikipedia.
The Wikipedia version includes the disclaimer that it “requires additional citations for verification.”