How to use it
- Look at the red line on the globe: the International Date Line as usually drawn.
- Type a place or press My location to see how far you are from it.
- Read the countries along it and the facts.
5 practical uses
- Plan a trip that crosses the line and gains or loses a day.
- Understand why Samoa and Kiribati changed sides.
- Teach time zones and the calendar.
- Check the date on the other side of the Pacific.
- See which islands sit next to the line.
The maths behind it
The line is a polyline of latitude and longitude vertices following the usual atlas drawing. The distance from a place to it is the haversine distance to the nearest point on the polyline, found segment by segment. The countries along it are found by intersecting the polyline with the country outlines. The zones either side differ by 24 hours: UTC+12 and UTC−12, or UTC+14 and UTC−10 where countries have chosen to be ahead.
What it is
The Earth's time zones run from UTC−12 to UTC+14, and somewhere the day has to begin. The International Date Line is that place: the boundary between the zones farthest ahead of Greenwich and farthest behind. Step west across it and the date jumps forward a day; step east and it goes back. It roughly follows the 180th meridian, on the opposite side of the world from Greenwich, because that puts it in the emptiest stretch of the Pacific.
Why it zigzags
No treaty defines it: each country picks its own date, and the line is just where their choices meet. It bends east around the tip of Siberia so all of Russia has one date, and west around the Aleutians so all of Alaska does. In 1995 Kiribati moved its Line Islands to the west side, so the whole country shares a day; that pushed a bulge of the line out to 150°W and made Kiribati's Millennium Island the first inhabited place to see each new day. In 2011 Samoa and Tokelau jumped west too, to trade on the same day as Australia and New Zealand, leaving American Samoa, 70 km away, a day behind. Tonga and New Zealand's Chatham Islands sit east of 180° but on the western side of the line.
Notes
The line drawn here is the usual atlas version; there is no single official one. The date is a matter of the calendar, not the clock: the time of day changes by an hour at every zone boundary, but the date only here.