The date is the first day of the period.

Namespace
cbc urn:oasis:names:specification:ubl:schema:xsd:CommonBasicComponents-2
Data type
udt:DateType
Example value
2017-06-01
Rules
Identifier/Error message Flag
BII3-T68-R020
The Period start date of the Catalogue line validity period MUST be before the Period end date.
fatal