[ibr-029]-If both Invoicing period start date (ibt-073) and Invoicing period end date (ibt-074) are given then the Invoicing period end date (ibt-074) MUST be later or equal to the Invoicing period start date (ibt-073).
ibr-029
cac:InvoicePeriod
(exists(cbc:EndDate) and exists(cbc:StartDate) and xs:date(cbc:EndDate) >= xs:date(cbc:StartDate)) or not(exists(cbc:StartDate)) or not(exists(cbc:EndDate))