The International Date Line – the place where the day begins and ends – is drawn rather “arbitrarily”. There are no international rules governing it, and its coordinates depend on the whims of governments.
Taylor Swift flies to Las Vegas from Japan and “gains” a few hours. The Hong Kong stock market closes when London opens.
These occurrences are due to the International Date Line – an imaginary line running through the Pacific Ocean, used to demarcate time zones.
Humans have agreed to follow time zones – a concept proposed by railway companies in the 19th century. But where exactly does the day start and end on Earth?
According to theoretical astrophysicist and postdoctoral scholar Cameron Hummels, from a physics perspective, there is no specific place where the Sun “rises first”.
However, to track time, humans have established a system of timekeeping, which includes time zones and the International Date Line to mark the start and end of the day at each location on Earth. “Thus, where the Sun rises “first” is essentially agreed upon along the International Date Line,” Hummels stated.
Nevertheless, there are no international rules governing the location of the International Date Line, and its exact coordinates depend on the willingness of governments to change it.
Maps attempting to depict it are never entirely accurate, and this line technically does not exist.
Not Officially Determined
The Earth rotates from west to east, with morning, noon, afternoon, and evening appearing sequentially in countries around the world. So, where should a new day start and end?
The idea of establishing a boundary where the day starts and ends has existed at least since the 1300s.
However, while the equator serves as a logical division between the northern and southern hemispheres, there is no clear place to divide the eastern and western hemispheres.
People in Warsaw, Poland, walk beneath clocks displaying time for cities around the world. (Photo: Bloomberg).
Mapmakers have long established their own east-west dividing lines, known as meridians – a term derived from Latin, meaning “midday”.
In 1884, 25 countries adopted a resolution calling for the establishment of a “prime” meridian at the Greenwich meridian – a town in the suburbs of London (UK) that houses the Royal Observatory.
This aimed to establish an international reference point for mapmakers, timekeepers, and railway schedulers. They were also determined to establish a “world day”.
However, it took decades for nations to accept the prime meridian and formalize time zones associated with Greenwich, according to Ian R. Bartky’s book One Time Fits All.
Despite the acceptance of the prime meridian, the specific location of the International Date Line has never been officially determined.
In 1921, the British Admiralty stated that no time point had “been established with certainty by any power or by international agreement.” This remains true over a century later.
“While the prime meridian feels inviolable, the International Date Line is not a meridian. It is drawn rather arbitrarily,” wrote Tim Montenyohl, the cartographer who mapped the date line, in 2018.
Some Countries Change Dates
Since the concept of the International Date Line is not enforced through an international treaty, countries and territories in the Pacific essentially have the freedom to decide which side of the line they wish to be on.
Some choices stem from political or commercial reasons.
Kiribati is one of the Pacific nations that unilaterally changed its date. (Photo: Josh Haner/New York Times).
Spain originally placed the Philippines – its colony in the 16th century – on the eastern side of the International Date Line, which caused the line to shift westward from the 180th meridian.
However, in 1844, the Philippines pushed this boundary back by declaring that December 31 of that year would be “canceled, as if it had never occurred.”
Some Pacific island nations unilaterally change their dates to simplify local timekeeping or to enhance trade relations within the Asia-Pacific region.
In the 1990s, Kiribati moved the date line to bend eastward past the 180th meridian to ensure that its easternmost islands shared the same time zone.
The Samoan Islands, now divided into Samoa and American Samoa, were located to the west of the International Date Line until 1892.
In that year, King Malietoa Laupepa was persuaded by American traders to adopt the American date zone (three hours after California) to replace the old Asian date zone (four hours before Japan). This change was implemented by repeating Monday, July 4, 1892 – American Independence Day.
In 2011, Samoa returned to the west of the International Date Line by eliminating Friday, December 30, 2011, from the calendar.
Emma Veve – an economist at the Asian Development Bank – who has worked in the Pacific Islands, stated that Samoa’s transition had commercial significance. It helped align the country with New Zealand’s business days.
While the media made a fuss, people continued with their lives, she remarked.
The Timeline Continues to Challenge Cartographers
For cartographers – and reporters – determining the International Date Line can be challenging.
For cartographers, pinpointing the International Date Line and the official timeline can still be difficult. (Photo: Shutterstock).
Mapmakers often create maps by referencing other maps, including time zones published by the CIA.
However, Montenyohl noted that creating a more detailed version is quite complex.
This is partly due to countries changing their time zones and the territory of a country extending 200 nautical miles from its land border.
The following case presents an interesting example.
In 2020, journalist Johnny Harris noticed discrepancies between two date line drawings around some of the Cook Islands in the South Pacific.
“Google says these islands are on Tuesday, while PacIOOS says they are on Monday, meaning a whole day apart,” Harris mentioned in a YouTube video.
So, which version is correct? To date, the answer remains unclear.
A spokesperson for the Cook Islands government did not respond to the inquiry. A representative from Google stated that the company’s timeline map has been updated since 2020.
Meanwhile, a data systems engineer at PacIOOS mentioned that their version is not the gold standard.
“We certainly are not experts or authorities on the timeline,” engineer John Maurer stated.
He added that PacIOOS has used a version similar to Wikipedia.
