I think Dan has serious memory problems as a result of his frequent use of radiation. Possibly he had a brain tumor and the treatment for it has caused his memory troubles or he hasn't had a treatment and the brain tumor itself is causing memory troubles. This explains why he doesn't remember Des coming to see him in 96. He says during that meeting "this future version of me, he referenced this meeting, so I would remember you coming to Oxford?" and Des says "no....maybe you forgot" and then Dan says "yeah right. how would that happen?" This statement just stuck out to me. Like, we will find out how that would happen in future episodes.