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The International Date Line was set at roughly 180° longitude, exactly halfway around the planet from the Greenwich Prime Meridian, following the 1884 International Meridian Conference in Washington, DC. That antimeridian was the obvious choice: it gave the world a fixed place where the calendar date changes, and it conveniently ran through the empty Pacific so almost nobody had to live on it.
Although the modern world is globalized and connected in countless ways, and advanced telecommunications depend on extreme accuracy of both time and position, this was not always the case. In fact, this obsession with accuracy when it comes to time and positioning is relatively new, and has only been around for the past few hundred years. Before that, it was simply impossible to coordinate when and where everyone was at a given time.
The Royal Observatory at Greenwich, just outside London, was founded in 1675, and a meridian running through it was used for British naval navigation from the late 18th century onward (Airy's transit circle nailed the modern line down in 1851). The International Date Line is a bit more mysterious, and since it is located in such an uninhabited area, most people don’t really understand what it is. Furthermore, most people don’t understand why it was chosen! Is it just an arbitrary line in the ocean? Or does it have some greater significance?
Short answer? It was chosen for convenience, and because it basically split the world in half.
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The Origins Of Timekeeping
Before clocks and instantaneous communication, telling the time was a rather simple process. When the sun was at its zenith (“noon”) and due south, that was the middle of the day. One full day passed between one “noon” and the next. Obviously, that system was somewhat faulty because cities on the same longitude, in certain areas, could have their “noon” occur at a different time. As the world began to modernize, clocks became more advanced and the need for accuracy of timekeeping increased (train schedules, telephone calls, etc.).
To solve this global problem, Canadian engineer Sandford Fleming proposed a system of 24 standard time zones in 1879, one for each hour of the day, and major nations adopted some version of it over the following decades. This corresponded to the global positioning system of longitude and latitude, which allowed for accurate positioning on Earth. Latitude is measured along the vertical axis, putting the Equator at 0 degrees and the poles at +/-90 degrees. However, longitude was a bit more of a challenge, as choosing where “0 degrees longitude” should be located was a major point of national pride.
Britain, however, had the largest navy in the world and the most colonies spread across the globe, so the British were in the best position to define the point on the map. By the time 26 nations gathered for the International Meridian Conference in Washington, DC in October 1884, most ships' charts already used Greenwich as their reference. The conference voted 22-to-1 (San Domingo against, France and Brazil abstaining) to make it official: the Greenwich Meridian became the world's Prime Meridian, sitting at 0 degrees longitude and running right through the suburbs of London. That helped standardize time and location, since most time zones are 15 degrees of longitude wide (360° ÷ 24 hours).
The Birth Of The International Date Line
Now, the world already had an understanding of how days and time worked (namely, the day and date changed at midnight), based on the time zones set in Greenwich. However, it was also necessary to have a demarcation of the “date change” somewhere else in the world. Since there is technically a “rolling date line” as midnight occurred in every different time zone, there needed to be a set location where a new day technically began. Thus, an International Date Line was required.
Crossing the IDL westbound advances the calendar by one day, and crossing eastbound rolls it back a day (the clock barely moves; it's the date that jumps). So if you step over to the western side at 2pm on May 1, your watch suddenly reads 2pm on May 2; step back east, and you're back in May 1 at 2pm. The reason we needed an international date line is because a rolling date line would never allow the date to change! That may sound crazy, but as you circumnavigate the globe using only the time zones to progress the date, the “math” doesn’t work out. Basically, if you traveled east around the world (the direction the Earth spins) using only the local time zones, you would eventually gain a day on someone who stayed in the same place.
The international date line is in place to separate two separate calendar days, essentially telling the world that the date changes at the IDL, and then at every subsequent place at midnight. In the eastern hemisphere, to the left of the line, the date is one day ahead than in the western hemisphere, directly to the right of the line.
An Arbitrary, But Clever Location
Given the strange nature of dates on opposing sides of the line, choosing a place for it on Earth had to be done quite carefully. It was only a matter of coincidence that the best spot for it was also on the opposite side of the planet from the Greenwich Meridian. The IDL is at 180 degrees longitude, Greenwich is at 0 degrees longitude, and there are 360 degrees of longitude on the planet.
The IDL was chosen because it runs right through the heart of the Pacific Ocean, in a relatively straight line, although it does zig-zag around eastern Russia and the Aleutian Islands of Alaska so that each country sits on a single calendar day. Two more recent kinks are worth knowing about: in 1995, Kiribati pushed the IDL east of its Line Islands so the entire country could share one date (the eastern bulge now reaches ~150°W, and Kiribati's Caroline Island, renamed Millennium Island, was the first inhabited spot to see the year 2000). And in 2011, Samoa and Tokelau jumped to the western side of the IDL overnight, skipping 30 December 2011 entirely, so they'd share weekdays with their main trading partners, Australia and New Zealand. The upshot is that Samoa and American Samoa, only about 125 km apart, now sit 24 hours apart in calendar terms. Cutting through the middle of the Pacific Ocean was wise, as few people will ever be in a position where they can hop from one date back to the next, thus preventing any major confusion.
For the most part, the IDL is accepted around the world because it gives some order to our timekeeping and calendar coordination. That being said, the IDL is not a part of international law (it’s a cartographic convention, and countries are free to observe whatever dates they choose), which is exactly why Kiribati and Samoa could legally yank the line around. For all of our prospective time travelers out there, we may not be able to live the dreams of H.G. Wells, but if you sail out into the middle of the Pacific and cross the IDL eastbound on the afternoon of July 14th, you’ll suddenly find yourself back in the afternoon of July 13th. Who needs a time machine when you have a sailboat and some time to kill?
