what is the {start time} for {Frankfurt} has {instance of} as {city} ?
When was the city of Frankfurt established ?
When was the city of Frankfurt founded ?
SELECT ?v WHERE { resource:Frankfurt_am_Main rdf:type ?m . FILTER ( CONTAINS ( ?b , 'http://dbpedia.org/ontology/City' ) ) . ?m rdf:type ?b . ?m property:start_time ?v }
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q1794> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P31> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> <http://dbpedia.org/ontology/City> .
?statement <http://www.wikidata.org/entity/P580> ?value. 
} 

SELECT ?value WHERE { wd:Q1794 p:P31 ?s . ?s ps:P31 ?x filter(contains(?x , 'http://dbpedia.org/ontology/City')) . ?s pq:P580 ?value}