Rental Check in and out with Time calculations

Rental Check in and out with Time calculations

Hi I'm trying to work out a way to get a form for my Ski and Snowboard Rental department up and running.  I'm stumped on how to calculate the number of days the guest will have the rentals out.  The part that is tricky is that we also calculate what time they take the rentals from the store and when they return them.  Let me use a weekend as an example.  Rentals can be picked up after 3 pm on a Friday, used that Saturday and returned before 3 pm on that Sunday. I do not count the pick up day (Friday) or the drop off day (Sunday) as a rental day, so the guest will only be charged ONE rental day.  

I have put into place two Time/Date fields on my rental form.  One field is for check out and the other is for check in.  any ideas on how to make this work.  Thank you.