PunditLisa–my assumption is it was tradition for them to go to that restaurant on their anniversary. As far as time goes, perhaps they had a “usual” time.
Or, and this is the more complicated explanation, perhaps Malcolm assumed he was late. My understanding of his existence was that he didn’t live in the normal constraints of time as we know it. If he doesn’t notice not having a conversation with his wife in months, or that the basement is locked, then he probably doesn’t notice that they hadn’t planned their anniversary. He probably remembered, arrived, and assumed–since she was there before him–that he was late.
Or something like that.
Teaching: The ultimate birth control method.