As far as I can see, the due day is either the starting day, or it's the next day, which is meant to be calculated, using that string value. Working out the next day based on that string is entirely the requested task.

I think everyone who is getting pulled in to this question is realising the same thing - this system is unworkable the way it's been designed, the broad variety of potential inputs needs to be standardised, put in it's own table or group of tables, and turned in to something it's possible to write reasonable SQL against.