When it was in theatres last fall this film caught our attention, and at the beginning of the new year we took a quick look at the time travel elements; it is now available on video, so we are looking a bit more deeply at the problems. A brief summary of the storyline is covered in the second of those linked articles, so we will dispense with it here.
The essential time travel element is that the central character and narrator Tim is told by his father that all the men in their family have always had the ability to time travel. It is very like what we know of Butterfly Effect, in which it is evident that Evan's father had the same ability (and in Butterfly Effect 2, where it appears that Nick's son may also have the condition). Here, though, Tim specifically asks about butterfly effect problems, and his father gives him the fairly non-committal answer that so far they've never had a major problem. He does fail to mention one problem that falls into this category, which arises later in the film, but we'll get to that.
This tells us that time travel has been happening for generations within this family, and that there have been probably hundreds of trips to the past, most of which were intended to change the past and succeeded in doing so. We thus need to determine whether there is a credible time travel theory in which this is possible. It is patently obvious that fixed time theory does not work here; we see Tim and his father change the past several times. We considered in connection with the Butterfly Effect franchise whether such a system would work under multiple dimension theory, and found it problematic; it is more so here.
When someone travels to the past in another dimension, whether a pre-existing parallel universe or a newly-created diverging one, he leaves the present universe and never returns to it. In this case, he does not duplicate himself, because he uses the body of his doppelganger. He does abandon the universe in which he started. In the Butterfly Effect franchise, that meant he was creating a lot of horrible universes. In this case, though, we have the obvious problem that there are two people altering the past, neither of whom is aware of changes made by the other. If Tim's Dad left for another universe, Tim would be aware that his Dad had left. It seems rather evident that they are changing their own histories, and the histories of those they know in their own world.
We thus need some form of replacement theory. That allows us to change history in our own universe. However, we have some serious complications to consider if that is the case.