Just for discussion I have to bring up the idea again of what qualifies as science fiction. See, I didn't remember any SF elements in Donnie Darko, just that it used the same retro-time-continuity / plot device as was used in The Last Temptation of Christ - and no one would classify that at a SF or time travel movie. (Coincidentally, a movie marquee is seen in Donnie Darko advertising The Last Temptation of Christ!)
Other time travel stories such as A Connecticut Yankee in King Arthur's Court or About Time involve time travel, yet there is no device or theoretical explanation as to how time travel occurs. Thus these stories aren't considered science fiction because there is no element of science behind them - they could just as easily be explained by magic or the supernatural will of God.
You've got a hold of several different narrative devices, there.
Donnie Darko is more of a Möbius strip story, that folds into itself. What makes it science fiction is not the time travel alone, but that it is the effect of a vortex.
The Last Temptation of Christ as well as
Brazil and
Jacob's Ladder employ a literary device like the Ambrose Bierce short story "An Occurrence at Owl Creek Bridge", which is not science fiction and is also not a Möbius strip. These stories have characters envision an imaginary alternate fate for themselves in the instant before death. That is not what happens in
Donnie Darko. In Mark Twain's
A Connecticut Yankee in King Arthur's Court the time travel happens after a blow to the head, so it is most easily interpreted as an unconscious dream in that satire, which is also not what happens in
Donnie Darko. In
About Time the ability to time travel is inherited at birth, which would make it fantasy and not science fiction and, again, is not the narrative in
Donnie Darko.
That you personally don't consider
Donnie Darko science fiction is A-OK. The people who voted for it did, and just about every source where you can look it up classifies it as sci-fi. But whatevs.