[BR-29]-If both Invoicing period start date (BT-73) and Invoicing period end date (BT-74) are given then the Invoicing period end date (BT-74) shall be later or equal to the Invoicing period start date (BT-73).
Stored validation assertion
(ram:EndDateTime/udt:DateTimeString[@format = '102']) >= (ram:StartDateTime/udt:DateTimeString[@format = '102']) or not (ram:EndDateTime) or not (ram:StartDateTime)