what is the {start time} for {San Antonio} has {head of government} as {Julian Castro} ?
When was Julian Castro the head of government for San Antonio?
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select distinct ?value  where {
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?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P6> .
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} 

When did San Antonio have Julian Castro as their head of government?