Period

The properties and their acceptable values are outlined below:

Property	Man.	Card.	URI	Value
label	rec	0..1	skos:concept	xml:string
startDate	rec	0..1	schema:startDate	xsd:date (ISO 8601)
endDate	rec	0..1	schema:endDate	xsd:date (ISO 8601)

As stated, the Period class consists of only recommended properties. However, when providing a Period for your dataset atleast one of these recommended properties must be provided. An empty Period class is considered invalid.