How can I increment a date field based on it's own value?

Thanks for the feedback.  I probably wasn't clear enough with my original post.  The Due-Date is a calculated date to indicate when the task is next due (let's say 1/20/17).  The Last-Completed-date is a date field that the user updates when they complete the task.  Once they update the Last-completed-date (let's say to today 1/12/17), I need to increment the Due-Date field based on the frequency (monthly for example) which would then change the Due-Date to 2/20/17.  Does that make sense?