I think the whole film has been the Joker telling the his story to the Dr in the white room. But because he's the Joker a lot of it is warped and not really what happened, it's his version of what happened. This would excuse a lot of the slow mo dancing with music, the part at the end where he is rescued and worshipped, the relationship which was obviously a fantasy (it was either that or some of the poorest writing in cinema) and various parts which are a bit jarring - sleeping in the fridge, the way the talk show plays his awful stand up, that would never really happen.
Near the beginning there is a brief flash forward to him banging his head on the door of that white room. I think that's because his other doctor asks him about getting out of the asylum, it brings him out of it for a split second. He looks older in that end scene, a bit of a grey beard forming. I think this scene takes place at a time where the Batman exists because he mentions sharing a joke, then there's a flash cut to Bruce Wayne next to his dead parents, then it cuts back to Joker and he says you wouldn't get it. I think he's seeing the irony that he created Batman that night.
I liked how it tied into Bruce Wayne and the murder of his parents by one of his followers. Loved the 70's Saul Bass Warner Bros. logo at the start. I really went into this film knowing nothing, and it was a surprise that is was set in the 70's or early 80's.