In Back To The Future, Marty calls to Doc's house to try to get his help, explaining that he's from the future. Doc flat out refuses to believe him, responding with sarcasm and incredulity, laughing at the notion of a celebrity president of the US, before running out of his house and down to his workshop / garage and slamming the door in Marty's face.
But why? While this might be an appropriate response from your average scatterbrained scientist, Doc had just that very day come up with the idea for time travel! Surely he should be completely open to the idea that someone could visit him from the future, especially with it so fresh in his mind.