The International Date Line Is ‘Pretty Arbitrary.’ Here’s Why.

Taylor Swift flies to Las Vegas from Japan and gains several hours. Hong Kong’s stock market closes as London’s opens. A clock on a remote Pacific island strikes midnight 24 hours early on a politician’s order.

None of those times are empirical scientific facts. Humans 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 on earth do days begin and end? The short answer is that Mondays become Tuesdays at the international date line, a boundary that runs through the Pacific Ocean.

The longer answer is that no international rules govern the location of the date line, and its exact coordinates depend on the shifting whims of governments. Maps that attempt to depict it are never quite right, and the line itself technically does not exist.

Confused? Here’s a primer.

People have been talking about this for centuries.

The idea of establishing a line where days begin and end has been around since at least the 1300s. 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 ones.

Mapmakers long chose their own east-west dividing lines, which are called meridians, a word derived from the Latin for “midday.” In the absence of an international standard for when days began or ended, navigators on long sailing 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 the English explorer and pirate Francis Drake described a ship arriving on a Sunday. But in “the ordinary reckoning of those that had stayed at home in one place or countrie,” it was already Monday.

The date line’s location was never settled.

In 1884, 25 nations passed a resolution calling for a “prime” meridian that set zero degrees longitude at Greenwich — a town on London’s outskirts that had a royal observatory — in order to establish an international reference point for mapmakers, timekeepers and train schedulers. They also resolved to establish a “universal day.”

But it took decades for many countries to accept the prime meridian and to formalize Greenwich-linked time zones, 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 settled.

In 1921, the British Admiralty, which managed the United Kingdom’s naval affairs, said that no date line had “ever been definitely laid down, either by any one power or by international agreement.” That is still true more than a century later.

“While the Prime Meridian feels sacrosanct, the international date line isn’t 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 concept of the international date line is not enforced by an international treaty, countries and territories in the Pacific are essentially free to decide which side of it to place themselves on. Some have switched sides for political or commercial reasons.

Spain initially put the Philippines, its colony from the 16th century, on the eastern side of the time change. That essentially forced the date line to dogleg west from the 180th meridian. But in 1844, the Philippines moved the line back by declaring that the day Dec. 31 that year would “be dropped, as if it had really passed.”

Some Pacific island nations have unilaterally moved the date line in order to simplify local time-keeping or to boost trading relationships within the Asia-Pacific region.

In the 1990s, Kiribati moved the line east across the 180-degree meridian to include its easternmost islands. In 2011, Samoa — which, at the urging of American traders, had hopped across the same meridian in 1892 by observing the same Monday twice — skipped back by cutting a Friday.

Emma Veve, an economist at the Asian Development Bank who has worked in the Pacific islands, said that Samoa’s switch made commercial sense because it put the country in the same business day as New Zealand. While the news media made a fuss, she said, people went on with their lives.

The dateline still challenges map makers.

For map makers — and reporters — the international date line can be hard to pin down.

Cartographers typically map it by consulting other maps, including a time zone one published by the Central Intelligence Agency. But making a more granular version is complicated, Mr. Montenyohl said. That is partly because countries change time zones; digital maps tend to reflect flaws of the predigital ones they were based upon; and a country’s territory extends 200 nautical miles from its land boundaries.

“It very quickly kind of, like, breaks your brain if you get too deep into the weeds,” he said.

Here’s a fun example.

In 2020, the journalist Johnny Harris noticed a discrepancy between two renderings of the date line around some of the Cook Islands, in the South Pacific.

“Google says that these islands are on the Tuesday side, the one-day-ahead side, whereas PacIOOS says that these are on the Monday side, meaning the one-day-behind side,” Mr. Harris said in a YouTube video, referring to the Pacific Islands Ocean Observing System, a nonprofit based in Hawaii.

So which version is right? It is still not entirely clear.

A spokeswoman for the Cook Islands government did not answer the question. A spokesman for Google said only that the company’s maps of the date line had been updated since 2020. And a data systems engineer at PacIOOS said that the group’s version was not a gold standard.

“We are certainly no experts or authority on the dateline,” said the engineer, John Maurer. He added that PacIOOS has used the same version as Wikipedia.

Wikipedia’s version includes the disclaimer that it “needs additional citations for verification.”