The milk expired two days ago. (expired is being used as a past tense verb here) The milk is expired. (...but here, expired is being used as an adjective)

“Expired” works best with something like a driver’s license. My driver’s license is expired. My driver’s license expired two days ago. Many people would use it for milk. To me personally it sounds a little clumsy or inaccurate to say “this milk is expired” but I think it’s pretty common. I’d more likely refer to a carton of milk’s “expiration date” or “sell-by date” being past. Is the milk in the fridge still good? Its sell-by date was a week ago. Let’s toss it.

The first one is correct, but instead, you could also say 'this milk HAD expired two days ago'.