How to incorporate a calendar in order to calculate a proper fee for items I will be renting out?

I'm struggling to figure out a way of applying a conditional formula to calendar widget and Square payment app. 

The form I created is designed to collect reservation information and payment for items a customer wants to rent for a period of time.

For example: a customer who rents and return a beach chair on the same day will pay 1 day fee, but any item returned the following day will incur an additional day's fee. 

How can I capture that on the form? I appreciate your help.