Folks have reached your conclusion way before you. This is a basic idea from early Christian thought. It is most often associated with St. Augustine. Catholic dogma is that God is necessarily eternal, in the sense of not being subject to time.
Then again, an omnipotent God can have His cake (His Cake?) and eat it, too. God can also choose to participate in time. He can even voluntarily be subject to it, in order to fully understand the limitations He has placed on others.
In fact by some notions of 'fully understanding', He would really need to do so -- which for C.S. Lewis and others, motivates the need for the central drama in Christianity. To directly experience the full range of human emotions in a first-person way, He might choose to live at one of the most annoying times in history, to be locally famous for a short time, and then to be mistaken for a criminal, betrayed by his followers, and killed in one of the more painful ways available, losing faith in His own power at the end.
Epilog: I tried to end it there, but I just couldn't. Let's dig around a bit in that last part. The very last words Jesus says in the Gospels while he is still human (well, just before "This is over.") is "O God! O God! Why have you forsaken me?" So this is where the canonical answer to your question jumps the shark, in a way that obviously scarred Kevin Smith as a child, and made him really funny.
So at this point, God has made a plan. He is going to become entirely human for 33 years, experiencing the true joys of free will and then die painfully, experiencing complete betrayal. He leaks enough of the plan ahead of time that he has to do it because he will be tempted to deviate and make it easy on himself when he has become human, selfish and weak. He has made himself just famous enough that this is all going to be passed down and recorded a century later and compared to the previews. He has played that out down to the last second, making only minor variations (pissing off some bankers, healing a woman by accident, fixing a severed ear here or there).
But no one can really monitor someone else's internal experience. If he wanted to cheat, right here, at the very end, and save himself the anguish of actual death, that could not be recorded. So at the very last moment, he changes his mind. (Maybe he will marry Mary M and run off and have some kids -- since I have already done one movie reference.) But then he finds out, evidently to his own surprise that he can't. His will is not done, so that His Will be done... He experiences actual powerlessness and feels betrayed by his own design. But that is really necessary, because he wants to have perfect first-perfect empathy on Judgment Day.
So the dogmatic answer is that God is omniscient and therefore eternal. But since God also temporarily had the human version of free will, he did experience what it was like to change his mind, and in fact to change his mind in one of those horrible situations where it was just too late and it didn't help, because he was so far down the rabbit hole that he could only despair of not having changed his mind sooner. Because that is really the worst.