I think maybe your process is a bit flawed here. Why do you need to determine when you might have been able to produce something in the past? It seems like you should have a production history table that maintains when something is made instead of trying to back into a possible date. Also, do you only ever produce one item at a time? I don't know what is being made but typically a manufacturing process includes a quantity to manufacture. The quantity of course is not needed when making larger items like cars or other items that can't be done in batches.