Your location:

Your location

Profile Login

Skip Navigation LinksHome > Learn > What Is the International Date Line for?

What Is the International Date Line for?


Ever heard of the International Date Line or IDL? What about the Greenwich Meridian, or Prime Meridian as it's called?


The International Date Line is an imaginary boundary that runs from east to west along the 180-degree meridian. Depending on which direction you travel - east or west - it's exactly the same distance away from the Greenwich Meridian.


The Greenwich Meridian, if you don't already know, is an imaginary line that runs North to South and is passes through Greenwich, London. Essentially, it represents the zero-degree mark and separates the east and west hemispheres of the Earth. Every longitude line east or west of this is measured in terms of its proximity to the Prime Meridian. Adversely, the Equator is what is used to measure latitude.


What Is the International Date Line for?


The International Date Line exists for a specific reason. It marks the time zone border where the date is actually changed by a whole day. On most maps, it is shown as an uneven black line because there are some locations where the border has been shifted. This is to keep countries in that area from being divided into separate time zones, such as Polynesia.


Although it would seem that way from the description, the time difference between countries on either side of the line is not exactly 24 hours. It depends on local time zone factors and varies depending on where in the world the divide is located.


To explain why it exists in simpler terms, let's say you're going to travel around the world and the International Date Line doesn't exist. When you move into a new time zone you must either subtract an hour if you're going east or add an hour if you're going west. Therefore, by the time you pass through all of the time zones, you'll have jumped ahead - or fallen back - a day from the actual calendar date. Things would get pretty messed up if we didn't account for this shift.


The International Date Line prevents the date from being uncoordinated with the real calendar. If you cross the IDL during travel while you're moving in an easterly direction, you must subtract a day, but if you cross the date line moving in the opposite direction - west - then you must add a day. Ultimately, it helps keep everyone across the world synced up with the real time.


Related Articles:

What is Terrestrial Time?

What is UTC?

What is a Time Zone?

Select A Location

Subscribe to our Newsletter

Add Clock To Your Website