Taylor Swift flies to Las Vegas from Japan and gains several hours. The Hong Kong stock market closes while the London one opens. A clock on a remote Pacific island strikes midnight 24 hours early on the orders of a politician.
None of those times are empirical scientific facts. Humans have just accepted observing time zones, a concept promoted by railroad companies in the 19th century.
But time zones have physical dimensions. So where exactly do days begin and end? The short answer is that Mondays become Tuesdays at the international date line, a border that runs across the Pacific Ocean.
The longer answer is that there are no international rules governing the location of the date line, and its exact coordinates depend on the changing whims of governments. The maps that attempt to represent it are never entirely correct and the line itself technically does not 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 at least since the year 1300. But while the Equator is a logical divider of the northern and southern hemispheres, there is no obvious place to divide the eastern and western hemispheres.
Cartographers long chose their own east-west dividing lines, which are called meridians, a word derived from the Latin meaning “noon.” In the absence of an international standard for when days began or ended, sailors on long voyages had to decide for themselves how to account for the time they were losing or gaining.
A 16th-century account of a voyage by English explorer and pirate Francis Drake described the arrival of a ship on a Sunday. But in “the ordinary calculation of those who had stayed at home in a place or country,” it was already Monday.
The location of the date line was never established.
In 1884, 25 nations passed a resolution calling for a “principal” meridian establishing zero degrees longitude at Greenwich (a town outside London that had a royal observatory) in order to establish an international reference point for cartographers. , timekeepers and train programmers. They also resolved to establish a “universal day.”
But it took decades for many countries to accept the prime meridian and formalize time zones tied to Greenwich, according to the 2007 book “One Time Fits All” by Ian R. Bartky. And the physical location of that universal day (the international date line) was never formally established.
In 1921, the British Admiralty, which managed the UK’s naval affairs, said that “a date line had never been definitively fixed, either by any power or by international agreement.” This remains true more than a century later.
“While the prime meridian appears sacrosanct, 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.
Since the international date line concept is not enforced by international treaty, Pacific countries and territories are essentially free to decide which side to stand on. Some have changed sides for political or business 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 turn westward from the 180th meridian. But in 1844, the Philippines rolled back the line by declaring that the 31st day of December of that year “would be removed, as if it had really passed.”
Some Pacific island nations have unilaterally moved the date to simplify local timekeeping or to boost trade relations within the Asia-Pacific region.
In the 1990s, Kiribati moved the line eastward across the 180-degree meridian to include its easternmost islands. In 2011, Samoa (which, at the urging of American traders, had crossed the same meridian in 1892 observing the same Monday twice) fell back by cutting off a Friday.
Emma Veve, an economist at the Asian Development Bank who has worked on the Pacific islands, said Samoa’s move made commercial sense because it put the country on the same business day as New Zealand. While the media made a fuss, she said, people went on with their lives.
The date line still challenges cartographers.
For cartographers (and journalists) the international date line can be difficult to pin down.
Cartographers often map it by consulting other maps, including a time zone map published by the Central Intelligence Agency. But making a more detailed version is complicated, Montenyohl said. This is partly because countries change time zones; Digital maps tend to reflect defects of the pre-digital maps on which they were based; and the territory of a country extends 200 nautical miles from its land borders.
“It very quickly breaks your brain if you get too far into the brush,” he said.
Here’s a fun example.
In 2020, journalist Johnny Harris noticed a discrepancy between two representations of the date line around some of the Cook Islands in the South Pacific.
“Google says these islands are on the Tuesday side, the day side ahead, while PacIOOS says they are on the Monday side, that is the day side ahead,” Harris said in a YouTube video . referring to the Pacific Islands Ocean Observing System, a nonprofit organization based in Hawaii.
So which version is correct? It’s still not entirely clear.
A Cook Islands government spokeswoman did not respond to the question. A Google spokesperson would only say that the company’s Dateline maps had been updated since 2020. And a PacIOOS data systems engineer said the group’s version was not a gold standard.
“We are certainly not experts or authority on the date line,” said engineer John Maurer. He added that PacIOOS has used the same version as Wikipedia.
The Wikipedia version includes the disclaimer that it “needs additional citations for verification.”