Charlie did some more counting in his head. "Harry went missing on the twenty-second of February. They found him three days later and he was in hospital for about two weeks. So he came out the tenth or eleventh of March. Then he would have been missing again for a period from the fifth through the tenth of April. My guess is that he was--replaced--in February. That jibes with your comments about his drug habits."
And, Charlie thought, raised a lot of questions about how 'Harry' 'got his magic back' after Mathwall. And this might well be why Quent had killed Dudley; not because of the switch itself, but because the creature had changed after Mathwall.