what is the {start time} for {Ryukyu Kingdom} has {instance of} as {vassal state} ?
During which period of time was the Ryukyu Kingdom a vassal state?
Over what period was the Ryukyu Kingdom a vassal state?
SELECT ?x WHERE { resource:Ryukyu_Kingdom rdf:type ?u . ?u rdf:type ontology:vassal_state . ?u property:start_time ?x }
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q28025> .
?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://wikidata.dbpedia.org/resource/Q1371288> .
?statement <http://www.wikidata.org/entity/P580> ?value. 
} 

SELECT ?value WHERE { wd:Q28025 p:P31 ?s . ?s ps:P31 wd:Q1371288 . ?s pq:P580 ?value}