Before the big bang? If God exists outside of time then he cannot create anything at a certain time. If God created Big Bang then he did it at a specific time, this specific time (Big Bang) could have happened at any time in relation to God but if god is timeless then the Big Bang has never happened or it has always happened.
If God is outside of time then the moment the universe was created at that same moment the end was also created and everything in between. And how does this short time, billions of years, relate to a timeless God?
Another viewpoint: If God created Big Bang at a certain point it means that God made a choice, the choice was, to create it (obviously) . And choice means confusion, uncertainty, you cannot choose something 100% because that would no longer be a choice. This makes God human-like-minded and it indicates that God exists in time.
So:
1. God is timeless. which means He could not have created the Universe at a certain point which means that Universe existed (not necessarily this universe) always with God, which means Universe is also timeless.
2. God exists in time. Which means the Big Bang happened at a certain point. Which also raises the question who/what created God, for everything that exists in time was created and will be destroyed in time. That makes Him very human-like, as most people imagine Him, especially in the old days.