The obligatory one.
Jenna is an alternate universe Haruhi Suzumiya.Matt is Kyon. After a birthday gone wrong, her subconscious provides a slap to the face by warping reality.
Alternative ending interpretation:The dust didn't work, the movie ends with Jenna alone. The "flashback" is just her thinking how things would have turned out differently.
Variation on the above:The dust doesn't work in that Jenna isn't actually sent 17 years into the future; what it does do is let her see what she'll end up like if she keeps going the way she's going, especially in wanting to be friends with the Alpha Bitch Lucy. In other words she never left the closet but did get a revelation that straightened her out.
The whole thing is a Stable Time Loop created by Adult!JennaThink about what her mother said about not regretting mistakes. She says that by making mistakes, you can learn how to fix them. Here is how this works: Adult!Jenna, the conniving, backstabbing Alpha Bitch version, goes back in time when Teen!Jenna goes forward. She, in her state of confusion and anger at going back to her teen self, is the one who lashes out at Matt, ruining the friendship. This causes the "mistake" that Teen!Jenna, on the verge of making it herself at the party, needed to learn about in order to fix it. The rest of Jenna's life was through Adult!Jenna taking The Slow Path back to 2004. Thinking that she has to "fix" her life by becoming the Alpha Bitch, she creates the circumstances that Teen!Jenna finds herself in. Teen!Jenna learns of her possible future, and at the end of the film, goes back and applies her own "fix".