The International Date Line (IDL) is an imaginary line of longitude on the Earth’s surface located at about 180 degrees east (or west) of the Greenwich Meridian. 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 right of 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.
The shape of International Date line is zig-zag so that the territory of a nation could have same date for administrative ease. As it is visible from the figure that several Islands in the pacific ocean have been under the administrative control of different nations.