If movement through time started at the big bang, there was no movement prior to it,
That is a big "if" that we should not neglect because it designates that "no" time before the universe is an assumption or perhaps in the multiverse time runs in every direction could very well be and everything that could ever happen has already happened all at once in the now of no time who's to say really. Just like what came first the chicken or the egg neither if we narrow our concept of what "came first" even means. What if time in our universe runs in the opposite direction that we all mentally perceive it as running what test could we do to find out for sure?
What about the idea that we have a mirror twin universe where the arrow of time is in reverse of ours? Could be but how do you prove it? At this stage of the inquiry we must proceed on assumptions because of the natural prohibition on exploration outside of our universe.