Question

Why international date line is not a straight line like other longitudes?

Answer 1

the international date line is an imaginary zig zag line on the surface of the earth roughly following the 180 degree meridian. crossing the date line causes the date to be changed exactly by one day. crossing the line from East to West, one loses a day.

Answer 2

Answer:

The International Date Line can be defined as an imaginary line of demarcation on the surface of the earth that runs from the North Pole to the South Pole and demarcates the change of one calendar day to the next.

If the line was straight, then it could pass through some territories and island groups. This will result in the increase of a day on the same island and a country which would create a massive confusion among the civilians. To prevent this confusion, the line is deviated  roughly following the 180° line of longitude.