International date line is an imaginary line which marks the spot on Earth’s surface where each new calendar day begins. The date just to the west of the international date line is one day later than the date just to the east of the line.
The international date line follows the 180th meridian for most of its distance. The 180th meridian is exactly halfway around the world from Greenwich, England, a borough of London. Another imaginary line, called the prime meridian or Greenwich meridian, marks 0° of longitude at Greenwich. See Greenwich meridian.
The sun travels over 15° of Earth’s surface each hour. For each 15° east of Greenwich, the time is advanced one hour. For each 15° west of Greenwich, the time is set back one hour. At longitude 180° East, the time is 12 hours more advanced than Greenwich time. At longitude 180° West, the time is 12 hours behind Greenwich time. When it is noon Saturday in Greenwich, Saturday is just beginning on the eastern side of the international date line, and just ending on the western side of the line. As a result, there is a 24-hour time difference between the two sides of the 180th meridian.
A new date first begins on the western side of the date line. As Earth rotates on its axis, this new date sweeps westward around the world. The date covers the entire Earth in 24 hours. January 1 occurs first just to the west of the date line. As a result, people in New Zealand, on the western side of the date line, start celebrating New Year’s Day 22 hours ahead of the people in Hawaii, on the eastern side of the date line.
The location of the date line was established through general practice of the larger nations of the world. The date line varies from the 180th meridian in some places in order to avoid differences in date within countries or between islands. For example, the northeastern corner of Siberia extends nearly 10° east of the 180th meridian. If the date line followed the meridian exactly, this little corner of Siberia would have a different date from the rest of Asia. So the date line was moved to run east of Siberia. Part of the Aleutian Islands extends west of the 180th meridian. However, the date line has been curved so that all the Aleutians remain east of it. The date line jogs again at the Fiji Islands to keep all the islands on one side of the line.
