IDL stands for International Date Line.
International date line is a theoretical line of demarcation on the surface of earth that runs from the north pole to the south pole and demarcates the boundary between one calendar day and the next.
Formula to calculate IDL.
When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.
Suppose you were crossing from Fiji to which is on the west of the IDL going to Cook Islands which is on the east of the IDL, calculate what time and day on the Cook Islands if you left Fiji on 0800h on Monday.
Since we are crossing from the west to the east, we are going subtract 24 hours.
So we will take the 0800h and subtract 24 hours. To get 0800h but the day changes to be Sunday.