We should have a way to make auto-renewals and manual renewals follow different RenewalPeriodBase behavior. I'm working with a library that uses a mix of manual and automatic renewals. For items that auto-renew, the renewal happens two days before the item is due, so they want RenewalPeriodBase set to add time on to the existing due date so the patron gets their full renewal period. But for items renewed manually, they want RenewalPeriodBase set to calculate from the renewal date, though, so patrons can't renew early and lock in extra time with the item.