PREFIX :        <http://example/>
PREFIX xsd:     <http://www.w3.org/2001/XMLSchema#>

SELECT ?X (SAMPLE(?v) AS ?S)
{
  ?s :p ?v .
  OPTIONAL { ?s :q ?w }
}
GROUP BY (COALESCE(?w, "1605-11-05"^^xsd:date) AS ?X)