What is actually going on?

The understanding-part is the most different part of Donnie Darko.

In fact-it is true that the world is going to end. Donnie lives in the primary universe, which is in great danger. Another universe called the tangent universe has been created. What Frank really means when he speaks to Donnie is that unstableness in the tangent universe will destroy the primary universe. This will happen in 28 days 6 hours 42 minutes and 12 seconds. The apocalypse-date is written on Donnie’s arm, and Donnie himself crosses the days on his calendar until the final day.

In order to avoid doom day, Donnie has to complete his mission and save the primary universe. The jet engine works as his portal or wormhole. Therefore, the jet engine leads him at first into the tangent universe, and at the end of the film out of it and back to the primary world. When Donnie is put back to reality-to the primary universe, he gets killed by the jet engine.