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

SELECT * {
   :s :p :o {| :source [ :graph <http://host1/> ;
                         :date "2020-01-20"^^xsd:date
                       ] ;
               ?source [ :graph <http://host2/> ;
                         :date "2020-12-31"^^xsd:date
                       ]
             |}
}
