what is the {point in time} for {Free University of Berlin} has {employees} as {4200.0} ?
When did the Free University of Berlin have 4200 employees?
SELECT ?value WHERE { wd:Q153006 p:P1128 ?s . ?s ps:P1128 ?x filter(contains(?x,'4200.0')) . ?s pq:P585 ?value}
select distinct ?value  where {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q153006> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P1128> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> <4200.0> .
?statement <http://www.wikidata.org/entity/P585> ?value. 
} 

What year did the Free University of Berlin have 4200 employees?