Question

why IDL is not a straight line

Answer 1

International Dateline is an imaginary line like all the other longitudes and is not a straight line just because of the reason to maintain the same date over a country. 

for the second part: 
24 hours=360 degrees 
1 hour = 15 degrees 
as 1 hour has 60 minutes so you get 
4 minutes = 1 degree 

and you were not doing anything wrong in that. 

just for your info in case you are interested: 
1. there are 24 zone in this world based on time each having a width of 15 degrees. 
2. the zero zone is 7.5 degrees west to 7.5 degrees east. 
3. zone times are generally considered as local time but cannot be used for calculation of sunrise/sunset/moonrise/meridian passages.etc. for calculacting above you must use correction as per the longitude of the place.

hope it helps.

Answer 2

the island in the middle of the pacific ocean make diffrent decesion regarding the time zone where they want to be