Dates between two dates within one month

Hi all, 

Couldn't find this answer but seems doable

I want to count the amount of days within one month between two dates. For example, if the start date is 8/1/2017 and the end date is 10/31/2017,  I want to return 31 because there are 31 days in August and when I run it again in September, I'd want it to return 30.

I tried using DATETIMEDIFF but that didn't include the part of days within a certain month. Is that doable?

Thanks!