I have looked at the posts on this topic using a Yes/No field.  But would be possible to do it with  DATEDIFF?   I have two simple fields.  A submit_date (timestamp)  and a deadline_date (text field as calendar popup).  The app requires a minimum of 30 days to complete a request.  So if someone submits a request (submit_date) and chooses a deadline date (Deadline)  that does not meet then 30 day minimum notice we need that Deadline date text to be read preferrably  (or the field background,