Message

If both Invoice line period start date (BT-134) and Invoice line period end date (BT-135) are given then the Invoice line period end date (BT-135) shall be later or equal to the Invoice line period start date (BT-134).

Context
cac:InvoiceLine/cac:InvoicePeriod | cac:CreditNoteLine/cac:InvoicePeriod
Test
(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))
Usage
/ ubl:Invoice / cac:InvoiceLine / cac:InvoicePeriod / cbc:EndDate
/ ubl:CreditNote / cac:CreditNoteLine / cac:InvoicePeriod / cbc:EndDate