Answer
Use Work_Time_Calendar_API.Get_End_Date, passing the starting date and a duration parameter expressed in working days — for example, starting from a Friday with a duration of 2 correctly returns the following Tuesday if the relevant Work Time Calendar has weekends excluded, rather than simply adding 2 calendar days. A Work Time Calendar can also be configured with exceptions for public/bank holidays, which this same method respects automatically based on however the calendar itself is set up.