Dennis @ PerfectForms
The reporting objects in a PerfectForms report cannot be used in behaviors. For this reason the approach will have to be a little different. Create a connection to your subject form. In the action have at least your date field and some identifying field. Place a table in your report with three columns (identifying field, date, and days since last result). When the report opens populate the first two columns of the table – all rows with the form connection. Next create a behavior that loops through the table and at each row uses a Set Field object with a Date Difference function to set the Result column – current row with the difference between the date column – current row and today’s date.
Unfortunately the table is not a standard reporting object so it will not set or respect the dynamic inputs of other reporting objects. You can however create a behavior to filter your table by specific parameters.