"closed" in this sentence isn't a verb, it's what grammar books call a predicate adjective. The verb is "is". Often this category of predicate adjective is an area of conflict between linguists, because we could also classify "is closed" as being a passive construction. As far as tense is concerned, in either case the result is the same - "is" is the verb and it carries the tense marker.
I hope that helps.
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