Although it's known that there was no "time" at the exact moment of the big bang, I've always taken that with a grain of salt; that could be used to infinitely freeze the timeline in philosophical debates, saying that "now" is impossible if there is any such time where a "then" was impossible, taking a chain of impossible "now's" and "then's", starting from any particular point where either is impossible. If it could be argued that time didn't exist at the big bang, then could it not be argued that there was no "now" at that point to be the next "now"'s "then", which would make every other point in time impossible. But, apparently that is self-evidently not the case, since we're here right now.
I've always understood the "timeless" moment of the big bang to be in relation to the lack of matter; if there was no matter for time to act on, then time has no effect and is essentially meaningless; "now" and "then" can't be measured as changes in entropy, so they don't exist. But, that means that you could go before the big bang if it was only an instantaneous moment of no-matter, and if there was a physical cause (in a pseudo-previous universe with matter) that caused the big bang. You could extend the timeline of this universe into the one that caused it.