what is the {start time} for {John D. Rockefeller Jr.} has {spouse} as {Martha Baird} ?
When was Martha Baird and John D Rockefeller Jr married?
When was John D Rockefeller Jr. the spouse of Martha Baird?
SELECT ?x WHERE { ?t property:start_time ?x . ?t property:spouse resource:Martha_Baird . resource:John_D._Rockefeller_Jr. property:spouse ?t }
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q363984> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P26> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> <http://wikidata.dbpedia.org/resource/Q15730684> .
?statement <http://www.wikidata.org/entity/P580> ?value. 
} 

SELECT ?value WHERE { wd:Q363984 p:P26 ?s . ?s ps:P26 wd:Q15730684 . ?s pq:P580 ?value}