FX-SCH-A-000059
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).
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
[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).
//ram:ApplicableHeaderTradeSettlement/ram:BillingSpecifiedPeriod
(ram:EndDateTime/udt:DateTimeString[@format = '102']) >= (ram:StartDateTime/udt:DateTimeString[@format = '102']) or not (ram:EndDateTime) or not (ram:StartDateTime)
Checked with Factur-X / ZUGFeRD BASIC