The International Date Line, Explained
An illustration of the International Date Line.
Credit: Dan Heim.

The International Date Line (IDL) is an imaginary — and arbitrary — line on Earth's surface that runs from the North Pole to the South Pole. When you cross the IDL, the day and date change. If you cross it traveling westward, the day goes forward by one, and the date increases by one. If you cross it traveling eastward, the opposite occurs.

The IDL is not a matter of international law, but it's one of the few standards embraced globally. The IDL is crucial for global interconnectivity, instantaneous communication, time measurement and consistent international databases. It's mostly about convenience, commerce, and politics. The IDL happened for much the same reasons as the emergence of the Internet — it works, and it makes life a little bit easier. Before discussing how and why the International Date Line came to be, we should first review the matter of keeping time.

Back in the days before mechanical clocks, time was measured mostly using sundials. People relied on the definition that "noon" was when the sun was highest in the sky, and due south. One "day" was simply the amount of time between two consecutive "noons." Most cities on the planet set their clocks to that cycle, and all was good — at least within any specific city.

Figure 1: The Sun at apparent (true) noon.
Figure 1: The Sun at apparent (true) noon.
Credit: Dan Heim.

The problem was, each city experienced noon at its own (apparent) 12:00 p.m. Depending on longitude, adjacent cities could have a time of, say, 11:45 a.m. or 12:15 p.m. displayed on their sundials. Near the equator, traveling westward by about 1,000 miles (1,600 kilometers) delays the arrival of noon by one hour.

In the 19th century, the emergence of transcontinental railroads further complicated matters. That century also saw accurate mechanical timepieces becoming widely available. Travelers found themselves resetting their watches by several minutes at every station to the east or west. This was inconvenient at best.

Also in that century, the emergence of telegraphy created time-keeping issues for commercial and military entities — the early adopters. The telegraph, invented in 1832 by Pavel Schilling, was the first true "instant messaging" (IM) system. It allowed communication over great distances using electricity, which moves (nearly) at the speed of light.

The telephone, patented in 1876 by Alexander Graham Bell, was the second such IM system. And of course, to use either system effectively, it's helpful to know the clock times at both the sender's and receiver's locations.

Before we explain how time zones solved these clock problems, let's do a quick review of latitude and longitude. Around 150 B.C., Hipparchus of Nicea, a Greek mathematician and astronomer, proposed a global grid of longitude and latitude lines to measure position. It was a coordinate system for locating points on the surface of a sphere. The vertical axis measured "latitude," and the horizontal axis "longitude." Though prescient, his idea languished for over a millennium.

During the Age of Discovery, beginning in the 15th century, cartographers saw the need for standardized latitude and longitude measurement. If your intent is to map or claim a geographic location, you need to describe its position unambiguously. Britain "ruled the waves" at the time, and took the early lead in this endeavor.

Portugal and Spain, the other major seafaring nations, were using their own systems, but eventually deferred to England. Latitude was less an issue than longitude, since there was no dispute about where the poles (latitude 90 degrees north and 90 degrees south) and equator (latitude 0 degrees) were located. However, the selection of a starting point for longitude measurement (the 0-degree meridian) was arbitrary. It was based more on national pride and convenience.

In 1851, England designated the Prime Meridian (0 degrees longitude) as the meridian running through Greenwich Observatory. They were the dominant seafaring nation in that era, had colonies around the globe, were using state-of-the-art mechanical clocks, and were scientifically qualified to establish a standard. You've heard the saying "The sun never sets on the British Empire." That was once true. England had colonies all around the globe, so it was always "daytime" somewhere in the British Empire. Britain had clout.

By the latter part of the 19th century, scientists, railroads and other emerging industries felt the need for a global standard of time. The first such system, using 24 standard time zones, was proposed by Sir Sandford Fleming in 1876. Sandford was a Scottish engineer, who helped design the Canadian railway network. His system wasn't officially sanctioned by any global entity, but by 1900 it spawned the adoption of the time zone system in use today. Nation by nation, the world bought into Fleming's idea.

Within each time zone, all clocks would be set to an average time that best represented where the sun was located in the sky. That time is called mean solar time. Sundials, by comparison, measure apparent solar time, sometimes called true solar time.

The time-zone process began in 1883 for the United States, when the nation was divided into four standard time zones. Each zone was centered on a meridian of longitude:

  • Eastern Standard Time (EST) at 75 degrees W (west of the Prime Meridian)
  • Central Standard Time (CST) at 90 degrees W
  • Mountain Standard Time (MST) at 105 degrees W
  • Pacific Standard Time (PST) at 120 degrees W

The United Kingdom had already started a similar process, and the rest of the world soon followed suit. By 1900, the global system of time zones we use today was fairly well established. Increasing global connectivity demanded some universal system of time measurement, and standard time zones were the answer.

Most time zones do not precisely follow meridians of longitude. They zig and zag as needed to keep islands, smaller countries and large metropolitan areas on the same clock time — an obvious concession to convenience.

Standard time zones are 15 degrees wide, since 360 degrees divided by 24 hours equals 15 degrees per hour. They are numbered by hour starting from the Prime Meridian (longitude 0degrees), which runs through Greenwich, England. The Greenwich clock shows what's called Greenwich Mean Time (GMT). The numbering system makes it easy to find the time in other zones.

For example, California, eight time zones west of Greenwich, is in a zone named Pacific Standard Time (PST). That zone is also labeled "GMT-8" or GMT+16." So if the time in Greenwich is 12:00 p.m., the time in California is 4:00 a.m. (12:00 – 8 hours).

Since 1972, GMT has been largely replaced by UTC (Universal Coordinated Time). When atomic clocks were invented in the 1950s, it became possible to measure time with an accuracy better than that provided by the rotating Earth.

GMT was an "average time" system based on telescopic observations from Greenwich Observatory. UTC, while synchronized to GMT, takes account of slight variations in Earth's rotation rate. Once in a while a "leap second" is added to (or subtracted from) the world's clock — that's a correction between GMT and UTC. Earth's rotation period can vary from exactly 24 hours by a fraction of a second either way, depending on geologic perturbations.

For example, as glaciers melt, there's a transfer of mass from higher latitudes toward the equator. As with a figure skater who slows his spin rate by extending an arm or leg, the law of conservation of angular momentum requires a reduction of the spin rate to compensate for this redistribution of mass. Scientists estimate a magnitude 9.0 earthquake in Japan in 2011 shifted enough mass away from the equator to shorten the day by 1.8 microseconds (0.0000018 s).

Astronomers must also consider the difference between apparent and mean time. That difference will depend on how far east or west one is located within a time zone, and also on the equation of time, which depends on the date. And then there's that confusing correction called Daylight Saving Time (DST). But again, to understand the IDL, we can ignore these complications.

We all know the day and date change at midnight, regardless of your location on the planet. But to use a global time zone system with an IDL, the day and date have to be separated at two locations — you can't split a circle into two parts with a single "cut." The solution was provided in 1884 by the International Meridian Conference (IMC), held in Washington, D.C., and attended by representatives of 26 nations.

The IMC selected the 180-degree meridian as the other "cut," not because it was directly opposite the Prime Meridian (any meridian could have been that other "cut"). The 180-degree meridian was chosen because it runs mostly through open ocean in the central Pacific, zigging and zagging to keep nearby nations on their own day and date. So the choice of 180 degrees was arbitrary, but it established the IDL in use today.

Although the IDL starts out in the middle of its UTC±12 time zone at both poles — exactly at longitude 180 degrees — for most of its length, it shifts to the east and coincides with the eastern edge of its time zone, which also zigs and zags. The bottom line is, this accommodation keeps the island nations of Oceania each on their own clock and calendar. But there are exceptions.

Just before midnight on Dec. 29, 2011, Samoans gathered around the clock tower in the capital city of Apia to celebrate the historic moment of hopping to the other side of the International Date Line. 

As the clock struck 12:00 a.m., the people of Samoa, along with their neighbors on the island of Tokelau, leaped ahead to Saturday, Dec. 31, 2011 — skipping Friday entirely. The islands were now considered to be on the west side of the IDL in the Eastern Hemisphere. Specifically, they changed their time zone from UTC-11 to UTC+13.

The decision was an economic one. Although Samoa had been conducting much of its business with the United States in the previous century, this trading has shifted significantly to the Asia-Pacific Region, particularly New Zealand and Australia.

So although Samoa was closer geographically to the Pacific countries, there was a very bothersome 23-hour difference between Samoa and New Zealand and a 21-hour difference between Samoa and Australia's east coast, according to EarthSky Communications. So in an effort to better synchronize their work weeks with their main trading partners, the two island countries countries decided to hop over the IDL.

In an article published on Dec. 28, 2011, in The Guardian, Samoa's prime minister, Tuilaepa Sailele Malielegaoi expressed the inconvenience with the previous IDL situation: 

"In doing business with New Zealand and Australia, we're losing out on two working days a week. While it's Friday here, it's Saturday in New Zealand and when we're at church on Sunday, they're already conducting business in Sydney and Brisbane."

This IDL transition was something of a homecoming for Samoans. More than a century ago, the country was on the western side of the IDL but decided in 1892 to move over to the eastern side to be closer to U.S. time. So for 119 years, Samoans witnessed the last sunset of the day and were the last to ring in the New Year — now they are one of the first.

Unfortunately, there will always be some inconvenience living so close to the IDL: Now there is a 24-hour difference between Samoa — located on the western part of the Samoa island chain — and American Samoa on the eastern side.

Tonga also preferred to be at UTC+13 (or UTC-11) for reasons of commerce and convenience. The Chatham Islands, almost 500 miles (800 km) east of New Zealand, sets clocks at UTC+12.75, creating an "orphan" time zone inside UTC±12. Fractional time zones are used in 16 locations around the globe. Countries simply choose what works best for them.

In the video above, study that first paused frame before you hit "play." It shows the IDL (white line) at the midnight point. For the sake of labels, let's say the green wedge represents the first hour of Saturday. The blue part of the Earth is still on Friday. The red part (which will appear later) will be Sunday.

That green wedge is the first time zone west of the IDL. West is clockwise as seen in this view from above the North Pole. Of note, this green time zone:

  • is 15 degrees wide, spans 1/24 of Earth's circumference, and one hour of time;
  • is centered on the 180-degree meridian;
  • extends from longitude 172.5 degrees to longitude 187.5 degrees;
  • coincides with the IDL along most of its eastern border;

The instant the IDL passes midnight, that entire time zone registers the start of a new day. All locations in a given time zone must be on the same clock time. There are some exceptions: nations (and regions within nations) that have opted out of DST, and those who have elected to use fractional time zones. But we can ignore that for now.

The model in this animation is idealized in many ways. Most importantly, all time zones are exactly 15 degrees wide, and centered on 24 evenly spaced meridians of longitude. Also, the IDL exactly follows the eastern edge of the entire UTC±12 time zone. This is not quite the way things are in the real world, but it greatly simplifies my model.

Now feel free to hit "play." Watch how blue Friday shrinks as green Saturday grows. Watch what happens when the IDL returns to midnight and the next day and date begins. You'll see red Sunday "unreeling" and replacing green Saturday as the Earth rotates. Use the slider to go back and forth and watch how that happens.

There are two things to notice about the IDL. First, at any point in time, there are two sequential days and dates in effect on the Earth. Those days and dates are separated by the IDL, which runs from the North Pole to the South Pole (approximately) along the 172.5-degree meridian of longitude.

Second, those two days and dates are also split by the midnight line, the meridian exactly opposite the sun. So there are really two "date lines" on Earth — one rotates with the planet (the IDL), and the other remains fixed at the midnight meridian. On opposite sides of both "date lines," the day and date are always different.

But wait. There appears to be an exception to that rule. The entire globe seems to be on the same day and date for one hour every day. It starts when the eastern edge of the UTC-11 time zone hits midnight. It ends when the eastern edge of the next time zone, the IDL (UTC±12), hits midnight. At that time a new day begins to unreel.

Watch the animation again if you didn't catch that. It only lasts for one hour, or about a second in the video. You'll see it twice, each time the IDL approaches midnight.

But, as explained before, this is an idealized model. Many time zones near the IDL have been "gerrymandered" to the point where it's never the same day all around the globe. In truth, it is for an infinitesimal "instant," — when the IDL hits midnight.

There are some exceptions to that scenario. For example, the Midway Islands are in UTC-11, and the Marshall Islands are in UTC±12. Check out this detailed map of the time zones in that area. If you use the Meeting Planner feature on the World Time Server for those two islands, you'll see they indeed share the same day and date for the last hour of the day, as my animation shows. You can see that result here.

There are other combinations that provide the same result. The bottom line is that time zones are so jumbled in this region that many "rules" are broken. For example: Crossing the IDL changes the day and date, but not the time. Exceptions exist for both parts of that "rule." That's why we need time zone maps and world time servers. Fortunately, GPS apps know all the rules and exceptions, so keep your smart phone on the right time, day and date wherever you travel.

Trick question. Since you have "crossed" the IDL, each foot would be in a different day. If you wore a watch on both hands, technically, they should be set to different days and dates. The question of what time those watches should be set to is not as easy to answer.

Depending where on the IDL you're standing, the times could be anywhere from equal to one hour different. Here is where Daylight Savings Time can mess things up, as some locations observe it and others don't. And then there's that fractional time zone complication.

But to "stand astride the IDL" isn't easy. Unless you're on a boat anchored at the IDL, there's really no place you could "stand" in the manner described except near the poles. Since the meridians of longitude converge at the poles, it's possible to walk across multiple time zones on an arbitrarily short hike. One kilometer from either pole, time zones are only 262 meters wide. If you were exactly on either pole, you could stand with one foot in all 24 time zones. 

Things get a lot simpler by using only a few time zones near the poles. Some scientific bases in Antarctica use New Zealand time (UTC±12), as that's a popular embarkation point for travel to Antarctica. Others set their clocks to UTC. Astronauts on the International Space Station do the same thing. The ISS is moving at the astounding speed of 4.7 miles per second (7.7 km/s). That's 5.7 times faster than a speeding bullet. The ISS makes one trip around the Earth every 90 minutes. So in 24 hours, the occupants experience 32 day and date alternations, and enjoy 16 sunrises and 16 sunsets. To keep things simple, their clocks are set to UTC+0.

Understanding the IDL is an exercise in arithmetic, and maybe some geometry. It's not magic, it's not physics and it's barely astronomy. It's all about setting arbitrary time standards on a rotating planet. Time, in that sense, is just another tool of a modern technological society.

One final historical note: During Magellan's 1519-1522 circumnavigation of the globe, his navigator diligently logged the passage of each day of their voyage. When they returned to home port, the day and date were off by one. It didn't take long to figure out how that error happened.

When you travel westward (opposite the direction Earth rotates), each day will be slightly longer than 24 hours — that is, if you measure your "day" as the time between two successive "noons." Over the three years of their voyage, those slight differences added up to an entire day. This was nearly three centuries before the IDL was established, but it demonstrated the need for day and date adjustments during global travel.

Thanks to science, that is all figured out now. In the 21st century, people take the IDL for granted. Trans-Pacific travel is routine, and we all know what happens when you cross the IDL. Now you know why it happens.

Dan Heim taught physics and mathematics for 30 years — more if you count his grade-school science club. Since 1999, he's been a freelance writer and creates educational computer graphics and animations. Dan is President of the Desert Foothills Astronomy Club in New River, Ariz. His weekly blog Sky Lights covers topics including astronomy, meteorology, and earth science, and questions from readers are encouraged. 

Additional reporting by Traci Pedersen, Live Science contributor.