I dont think it would be open after 5pm. It's not the valley, where they have the money to pay for a chef to stay late and have dinner. "Working late" is probably a massive anomaly for your average government employee, cause the pay is shit.
https://maps.app.goo.gl/iCZBTiWQUKSKU1LS9
From everything I've seen, though, it seems like there are 20+ shops inside the Pentagon and GMaps only lists a few, so I don't know how accurate that information is.