Actually, we are not certain time began with the big bang. We are also not certain that time has only one dimension. Modern physics postulate the universe can have many more physical dimensions than we can see. The reason we can only see three is because all of the senses we currently have access to, whether given by nature or by instruments we've thus far invented, ultimately relies on what physics call messenger particles that are constrained to move in our three dimensions.
It may be that time has several dimensions, and like physical dimensions we can only perceive a subset of them. In this case, perhaps the dimension we sense started with big bang, but other orthagonal to ours can't be said to have started at any point along our time.
It may be that time has several dimensions, and like physical dimensions we can only perceive a subset of them. In this case, perhaps the dimension we sense started with big bang, but other orthagonal to ours can't be said to have started at any point along our time.