"How long will it take to make, anyway?" said Harry as Hermione, looking happier, opened the book again.
"Well, since the fluxweed has got to be picked at the full moon and the lacewings have got to be stewed for twenty-one days ... I'd say it'd be ready in about a month, if we can get all the ingredients."
"A month?" said Ron. "Malfoy could have attacked half the Muggle-borns in the school by then!" But Hermione's eyes narrowed dangerously again, and he added swiftly, "But it's the best plan we've got, so full steam ahead, I say."
Why "could have attacked..." is said in the Past Perfect tense? Ron obviously meant the future while saying it.