Donnie Darko. My Theory.
At the beginning of the film, there is a dinner table scene in which Donnie's sister mentions Donnie's medication. This scene lets us know that he is mentally unstable in some way. This is later described as paranoid schizophrenia.
The night the plane crashes into Donnie's house is the 2nd Oct 1988.
That night, Donnie gets out of bed and takes a walk. This is where he meets Frank, the giant bunny rabbit.
This Frank, the one we see in this scene is in fact a ghost of the Frank we later see shot by Donnie. This ghost has come back through time to give Donnie a choice. He shows Donnie the future, right up until the moment he is sitting at the top of the cliff, watching the tornado (or portal).
Everything you see, from the point of Donnie meeting Frank on October 2nd, to the day after halloween (the cliff scene) is in fact a vision that the ghost of Frank is showing him.
At one point in the future (in this vision) Donnie is speaking to his science teacher about 'God's channel' and how you can not change the future events, no matter what you do. Donnie realises on the night of 2nd October, whilst Frank is showing him this vision, that what he is seeing (and what we assumed actually happens during the rest of the movie) is 'God's channel'. The ghost of Frank shows him the plane crashing into his house, which in fact happens roughly 6 hours after Donnie has taken his stroll to meet Frank; Donnie meeting Gretchin, the school flooding, Donnie burning Jim Cunnigham's house down, the halloween party, Gretchin being run over and Donnie killing Frank in revenge. Donnie knows that he has 2 choices. He can go back to bed and prevent all the above from happening (as he knows that there is no way to change what the future holds, as it is a pre-decided event), or he can take the path shown to him by Frank (the ghostly apparition).
Frank is the only character in this film who engages in any type of 'time travel'. What is shown in the movie, from the moment Donnie first meets Frank, to the cliff scene is something that actually happened in reality, however what we are seeing as the viewers is in fact the vision Donnie is seeing that night (2nd October).
Frank has gone back in time to prevent his own death. He knows, that by going back in time (as a ghost) he can let Donnie know what will happen had he gone 'sleep golfing' that night (thus preventing him from being in the house the night of the crash - thus taking the path Frank is showing him).
The scene at the end of the film, where Donnie is lying in bed is a replacement to where he goes 'sleep golfing', like an alternate ending.
The night he dies in the film (at the end), the 2nd October, is in fact the very same night you see him meet Frank(at the beginning) There is no time travel involved to take Donnie back to that night. The film leads us to believe that Donnie in fact followed 'God's channel' and flooded the school, burnt the house down, etc etc.
Let's say that this is what happened before the film we saw started, a kind of prologue.
* Donnie meets Frank (the "person", as the film leads you to believe) *Donnie floods school, burns house down, has a halloween party, Gretchin gets killed and Donnie shoots Frank. *Frank as a ghost goes back in time Lets say that all the above happened in a part of the movie we did not see.(the prologue - not included in the film) This is where we see the film starting: *Frank (the "ghost") lures Donnie to the place they first "meet". He tells Donnie all about the things that have happened(as above). Donnie see's it as the future. These were the scenes we saw as actually happening. Donnie in fact, does not go sleep-golfing that night. He goes back to bed.
The scene we see at the end of the film, with Donnie in bed, laughing and being killed by the plane is in fact, only 6 hours after we first see him meet Frank.
This is what I think actually happened: *Donnie goes sleep walking and meets Frank. Frank shows him the future (the flooding of the school, etc etc) *Donnie prevents this all from happening by going back to bed, thus preventing all the following events.
(The following events being the 'vision that Frank's ghost showed him that night, right up until he's back in bed laughing) The whole movie was set in one day. The dates flashing up on the screen, lead us to believe that the movie was progressing in time.
However, these dates were just a reference point given to Donnie by Frank, on the 2nd October.
If any of this is unclear, please let me know and I will do my best to clarify. Sorry for how long winded this theory is, but it's a pretty hard movie to make sense of.