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What Is the International Date Line (IDL)?

The International Date Line (IDL) is an imaginary line on Earth's surface defining the boundary between one day and the next.

Time Zone map showing the International Date Line.

The International Date Line (IDL) on the map.

The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, London, UK, the reference point of time zones. It is also known as the line of demarcation.

The Dateline Is Not Straight

The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half. On timeanddate.com's Time Zone Map, the IDL is shown as a black line (see image).

What Happens When You Cross the Dateline?

When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.

Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours. For example, if you travel the 1061 kilometers (659 miles) across the dateline from Baker Island to Tokelau you have to add 25 hours, or 1 day and 1 hour.

Three Dates at the Same Time

Every day between 10:00 and 11:59 UTC, three different dates on the calendar are in use at the same time on Earth. For example, our Time Zone Converter shows:

  • At 10:30 UTC on May 2, it is
  • 23:30 (11.30 pm) on May 1 in American Samoa (UTC−11),
  • 06:30 (6:30 am) on May 2 in New York (UTC-4), and
  • 00:30 (0:30 am) on May 3 in Kiritimati (UTC+14).

Changing Sides of the Dateline

The dateline is not defined by international law. Countries are free to choose the date and time zone that they want to observe.

For example, when the Republic of Kiribati gained independence from being a British colony in 1979 some of the islands were on one side of the dateline, and the rest were on the other. They corrected the anomaly in the eastern half of Kiribati by skipping January 1, 1995 and ever since Kiribati has been the first country to enter the New Year.

In 2011, Samoa changed the time zone from UTC-11 to UTC+13 by shifting the dateline to the west and removing December 30, 2011 from the calendar. They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.

Drawn up in 1884

The 180° meridian was selected as the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean. It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.

Topics: Timekeeping, Time Zone, Geography