But that's the question. First of all, you're saying that time begun the moment molecules started existing. But what made molecules start existing? Wasn't there time before the existing before moleculels? If not, then how did the molecules started exist if there were no time before to lead up to the existence of the molecules?
Secondly, you're saying that "before the beginning" is not within the arc of time. By what statement you're suggesting that time is only our imagination/fabrication/invention. Because how can we even decide what involves in what you call "the arc of time" if we don't even know what were before the arc of time... or what will be after?