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.

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.