Dan is a very up-to-date person who keeps up on the latest trends, and refuses to acknowledge anything that's over five years old.
Dan listens to disco, but he listens to other music, too.
When we first meet Dan, he is dressed in 1970s disco gear... because he's going to a fancy dress party and thought it would be funny. He consequently shows no unusual amount of interest in disco culture.
Which he unfavorably compares to disco. And he wears the leisure suit when he goes out clubbing.
Dan displays no unusual interest in disco culture in public — in private, however, it's another story...
Deconstructed: Dan will be missing out on a lot of things that may not be from his favorite decade, but are still good. He's also alienating himself from others and making himself the laughingstock of the town.
Dan learns that other music and fashion is good too. He can relate to others better...and they (usually) forgive his undying love for The Bee Gees.
Dan is blinded by nostalgia, but he's still pretty nice and easy-going about it; consequently, the people around him indulge his eccentricities and let him get on with it in good humour.
Played For Laughs: Dan's character only shows up when it's convenient for a joke, and anything dealing with how he manages the rest of the time is completely ignored to prevent Fridge Logic from kicking in.
Played For Drama:
Dan won't talk about or acknowledge anything that occurred after 1979. This keeps him from using computers, the internet, and many other facets of modern technology. Dan only uses calendars from the 1970's, so he shows up for appointments on the wrong day. People around Dan find him embarrassing to be around in public, and desperately try to get him to tone it down. Any attempts to question his dedication to this lead to violent and sometimes dangerous outbursts. Eventually, it's revealed that Dan underwent some trauma in the 1970's, and is psychologically unable to move on. The plot of the story is based around working through Dan's inner demons and helping him overcome his past.
Alternatively, it's not Dan who needs to learn something — his fascination with disco culture might be eccentric and taken to certain extremes, but is otherwise not harming him nor anyone else. It's actually the people around Dan who need to learn a lesson about not forcing Dan to conform with their own worldview just because they do not share his, and not being bullying Jerkasses just because someone else's taste in music and fashion is 'outdated' and different from their own.