My interpretation was when Frank the bunny (manipulated dead) said "The world is going to end in 28 days blah blah blah blah", he meant that Donnie's world, the tangent universe was going to end.
At midnight a Tangent Universe spins off of the Primary Universe, signified by the appearance of an Artifact; here represented by a jet engine. Tangent Universes are inherently unstable and will collapse in less than a month, taking the Primary Universe with it, if not closed off. Closing the Tangent Universe is the duty of the Living Receiver (Donnie), given super powers to perform this task. Those who die within the Tangent Universe are the Manipulated Dead (Frank and, according to the back of the book, Gretchen) who are also given certain powers, understanding of what is going on, and the ability to contact the Living Receiver via the Fourth Dimensional Construct (water). Everyone else in the orbit of the Living Receiver are the Manipulated Living, who are subconsciously drawn to push and prod the Living Receiver towards his destiny, closing the Tangent Universe and, apparently, dying by the Artifact.
So Frank was pushing Donnie to close off the Tangent Universe, or the Primary Universe would indeed end.
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