cac:InvoicePeriod[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).
Hitting this rule in production? Paste the invoice into the validator or wire the API into CI so it never ships broken again.