Because the sun rises around the world at different times, the time of day varies from one place to another. This did not cause much of a problem until people started travelling far and wide. That is when it became useful to know which day they were in. So it was decided to create an imaginary line to divide one day from the next. This is the International Date Line. It runs north-south, roughly following the 180 meridian. That is the line on the opposite side of the world to the 0 meridian, which runs through Greenwich. One or two islands get in the way and the line zigzags round these. It goes east of the Aleutian Islands on the north, then west of Fiji, Tonga and the Chatham Islands.
People going east cross the International Date Line and put their calendars back one day. People travelling west put their calendars forward a day.
Picture Credit : Google
