Calculate the due date of the bills :
30th april 2017 - 30 days in comparison to 30th april 2017 - 1 month
(explain in detail)

Dear Student,
In the first case, 30th April 2017 - 30 days :-
We will count 30 days, starting from 30th April 2017, which is going to end on 29th May 2017. Plus 3 days of grace will be given. 
So, the due date will be 2nd June 2017.

While, in second case, 30th April 2017 - 1 month
We will count 1 month, starting from 30th April 2017, which is going to end on 30st May 2017. Plus 3 days of grace will be given. 
So, the due date will be 3nd June 2017.

Hope this helps you!
Regards