Due Date Calculation in JDE 9.1

How we can update due date for the logic mentioned below. Invoice Date + Credit period (e.g.30, 45, 60 .... days) and then 15th of month needs to be updated in due date field. e.g. Invoice Date = 12th June 2015... Credit days = 30 days .....  Due date as per this = 11th July 2015 ..... but system needs to update Due Date as 15th July 2015 e.g. Invoice Date = 16th June 2015... Credit days = 30 days .....  Due date as per this = 15th July 2015 ..... but system needs to update Due Date as 15th