Question: When was the upgrade of the Parks-Baba Park paid?
SPARQL:
 select distinct(?date) from <http://linkedspending.aksw.org/pbw-ct> { ?obs qb:dataSet ls:pbw-ct. ?obs lso:pbw-ct-Description <https://openspending.org/pbw-ct/Description/upgrade-of-parks-baba-park>. ?obs lso:refDate ?date. }