The International Date Line is an imaginary line on the surface of the Earth that sits mostly on the 180º line of longitude in the middle of the Pacific Ocean.
The International Date Line lies on the opposite side of the world to the Prime Meridian (The Prime Meridian passes through Greenwich in London. It separates the Eastern and Western Hemispheres).
The International Date Line separates two consecutive calendar days.
Immediately to the left of the International Date Line (the date) is always one day ahead of the date (or day) immediately to the rightof the International Date Line in the Western Hemisphere.
People crossing the line from East to the West skip forward a day. Those crossing from the West to the East repeat the day.
If you flew from Tonga (on the Western side of the International Date Line) to Samoa (on the Eastern side) it takes two hours. But because you cross the IDL, you arrive 22 hours before you left!
The island of Kiribati used to be on the Eastern side of the International Date Line but is now on the Western side. of the line. It changed sides in order to be the first nation to see in the year 2000.