What is the international date line?
The international date line is an imaginary line that marks the spot on earth where each new calendar day begins.
Because of our earth turns on it axis, the sun is always rising and setting some place on earth, and the day is just beginning or ending.
So that the day would not be forever chasing itself around the world as it follows the sun, it is necessary to have one place where we can say the calendar day begins.
The place chosen to mark the beginning of a new day is an imaginary line that runs from pole to pole through the Pacific Ocean. It is called the International Date Line.
A new date begins on the west side of the date line. When it is Monday on the west side of the date line, for example, it is still Sunday, or one day behind, on the east side of the line.
As the earth turns the new date sweeps westward, to cover the earth in 24 hours. The International Date Line is not a straight line. It zigzags in several places so that it will not pass through a country and give the people two different dates at one time. – Dick Rogers