Fixed day - x number of days

Fixed day - x number of days

I have an issue in a form where I need to derive a new date based on a fixed date minus a variable number of days.

The formula field would look something like:
((Fixed_Date - variable_days)  /  (1000  *  60  *  60  *  24)).round(0)

However the issue of course is that the 'variable_days' field is STRING type data.

How do I workaround this?