If both delivery period start date and delivery period end date are given then the end date SHALL be later or equal to the start date.
cac:PromisedDeliveryPeriod | cac:OrderLine/cac:LineItem/cac:PromisedDeliveryPeriod(exists(cbc:EndDate) and exists(cbc:StartDate) and (cbc:EndDate) >= (cbc:StartDate)) or not(exists(cbc:StartDate)) or not(exists(cbc:EndDate))