This discussion you're asking for takes me some time to say/type. Therefore, I'll sum it up.
When the jet engine first falls into Donnie's room, it creates a rift in the normal universe/reality because Donnie was alive. Donnie needs to repair said rift by taking the course of the movie, sending the Jet Engine back in time and killing Frank, therefore rendering Frank unable to go back in time and telling Donnie to come outside, therefore killing Donnie, allowing time to pass normally.
The problem with the rift is that because Donnie survived, the Jet Engine couldn't exist inside the same universe/reality as him, a better description of the rift would be tangent universe, and those can stay open for a few weeks, (about four/28 days) until they collapse, destroying the normal universe.
Someone who knows this better than I please correct me, I have a bad case of ADHD and I forgot to take my meds this morning.