I'm not sure about the visitor center but the park itself closes when it gets dark. If you call the visitor center I'm sure you can get a quick and accurate answer.
Visitor Center: 9 a.m. – 5 p.m. weekdays. 9 a.m. – 1 p.m. weekends. (Closed Tuesdays)
At dark (right now probably at 7.30pm).
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