what is the {start time} for {Charles, Prince of Wales} has {position held} as {Prince of Wales} ?
When did Charles, Prince of Wales, become the Prince of Wales?
On what day did Charles, Prince of Wales, become the Prince of Wales?
SELECT ?v WHERE { ?r property:start_time ?v . resource:Charles,_Prince_of_Wales property:position_held ?r . ?r property:position_held resource:Prince_of_Wales }
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q43274> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P39> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> <http://wikidata.dbpedia.org/resource/Q180729> .
?statement <http://www.wikidata.org/entity/P580> ?value. 
} 

SELECT ?value WHERE { wd:Q43274 p:P39 ?s . ?s ps:P39 wd:Q180729 . ?s pq:P580 ?value}