What you describe makes sense to me. If the date field calculates the current date, then each time that PDF document is opened, the calculation would execute. If you want to preserve the date of when the PDF was originally filled out, you would need to add logic to populate a "this form was filled out on" date field from a (hidden) current date field. Make sure this logic only executes when the form is originally filled (set a flag or something in the logic).
