It comes from the supplier's Delivery Window. If the window type is Days After Cutoff, the date is the ordering cut-off of the report's Begin delivery date plus the window in days (if no cut-off is recorded, Begin date plus the window). If the type is Days Before Delivery, it is the Begin delivery date minus the window. With no delivery window set, the Begin delivery date itself is used. The date is generated when you open the preview, so pick the right Begin date before sending.