Issue coding in a flexible due date calculator

I'm working on making a contract that can, among other things, calculate a flexible due date based on a chosen date by the customer.
So if there's 11 payments left, and they want to make their first payment on the 13th of March, it can figure out their last payment will be due on the 14th of Agust.
Just from copying some code I got from a youtube video I made a version that technically works, but you need to be able to select the date from the dropdown calander.
What I need is to adjust it so that I can type in the dates manually into a text field and it can still read that and give me the correct date. That way it can still work outside of PDF readers and acrobat.

everything above NumberOfWeeks should be irrelavent since it's just figuring out what formula it needs to use to correctly figure out the date.
the text field it's getting it's information from "First_Payment_Date" currently has no javascript and looks like this