Question

Why the international date line is irregular in shape?

Answer 1

hey ,
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 linefrom East to West , one loses a day .
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Answer 2

The  IDL called a calendar line because it mostly runs through the sparsely populated central pacific ocean.

                                            It is irregular in shape because it is an imaginary line, like longitudes and latitudes, to avoid any confusion of date, this line is drawn through where the sea lies and not land.