what is the {start time} for {Nantes} has {head of government} as {Johanna Rolland} ?
When did Johanna Rolland become Mayor of Nantes?
When did Nantes begin having Johanna Rolland as head of government?
SELECT ?j WHERE { ?t property:head_of_government resource:Johanna_Rolland . resource:Nantes property:head_of_government ?t . ?t property:start_time ?j }
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q12191> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P6> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> <http://wikidata.dbpedia.org/resource/Q16037012> .
?statement <http://www.wikidata.org/entity/P580> ?value. 
} 

SELECT ?value WHERE { wd:Q12191 p:P6 ?s . ?s ps:P6 wd:Q16037012 . ?s pq:P580 ?value}