Other than your argument P3, the default is God as understood with the conventional properties assigned to God, God is not a simple being.
God is also known as the Absolute.
en.wikipedia.org/wiki/Absolute_(philosophy
Since God is Absolute, i.e. totally unconditional, God cannot be conditioned by time or any other conditions/variables. Thus your P1 and P2.
However I believe your conclusion is not complete,
Your conclusion should be;
God is impossible to be subjected to time.
Your argument would be better presented via this syllogism’
P1 Absolute [theistic] meant totally unconditioned of any variable [e.g. time]
P2 God is the Absolute
C1 Therefore God is unconditioned by time.
It is impossible for God to be conditioned by time.
There is nothing significant with the above.
What is significant is this point;
Is God a possibility to exists as real?
I have argued the point here
God is an Impossibilty to be real
viewtopic.php?f=5&t=193474&hilit=god+impossibility#p2683202