<obs1> a qb:Observation ;
  my:lastupdate '2017-12-31'^^xsd:date ;
  qb:dataSet <dataSet1> .

<obs2> a qb:Observation ;
  my:lastupdate '2016-12-31'^^xsd:date ;
  qb:dataSet <dataSet1> .

<obs2_1> a qb:Observation ;
  my:lastupdate '2017-12-31'^^xsd:date ;
  qb:dataSet <dataSet2> .

<obs2_2> a qb:Observation ;
  my:lastupdate '2015-12-31'^^xsd:date ;
  qb:dataSet <dataSet2> .

SELECT * WHERE {

  ?observation my:lastupdate ?datenstand ;
                 qb:dataSet <dataSet1>
} ORDER BY DESC(?datenstand) LIMIT 5