If seller is icelandic and invoice contains supporting description EINDAGI then the id form must be YYYY-MM-DD — Ef seljandi er íslenskur þá skal eindagi (BT-122, DocumentDescription = EINDAGI) vera á forminu YYYY-MM-DD.
ubl-creditnote:CreditNote[$SupplierCountry = 'IS'] | ubl-invoice:Invoice[$SupplierCountry = 'IS'](exists(cac:AdditionalDocumentReference[cbc:DocumentDescription = 'EINDAGI']) and string-length(cac:AdditionalDocumentReference[cbc:DocumentDescription = 'EINDAGI']/cbc:ID) = 10 and (string(cac:AdditionalDocumentReference[cbc:DocumentDescription = 'EINDAGI']/cbc:ID) castable as xs:date)) or not(exists(cac:AdditionalDocumentReference[cbc:DocumentDescription = 'EINDAGI']))