I have a query where I want to add number of days since last refill. How would I do this to ensure that the query knows weather there is 30 or 31 days in a month. For example, say I have a patient that on february 20th received a prescription with a 28 day supply. How could I builed a field where it will take the 28 days, add it to the 20th, and list the "due date" for the next refill?