But, he did get the knife to try to destroy the blockbuster, and he did talk his dad into flipping out at him But the outcome, if you noticed, didn't change. He went back and grabbed the knife, but then his mom found him and everything was the same. The
reason he grabbed the knife may have been different. We really don't know and it's not significant.
His dad flipping out didn't really make a difference in the future either except it gave him more knowledge of what was going on and what he needed to do. It didn't change the future. We don't know if that is what happened durring the original black out spell or not. Again, it doesn't matter because he got what he needed.
Basically, returning to the past where the blackouts occurred weren't always to change something. It was just to see what he missed, or what we missed during the first part of the movie.
I personally liked the director's cut ending better than the "hollywood" ending.