Time measures change or, as Aristotle would put it "motion".
Think about what a clock is. It's an attempt to make an obsesvable closed system (so that the clock isn't affected by things outside it) in which something happens (and something else counts the thing that happens - but that's not essential, We could hire a grad student do the counting for us if the system is observable.)
Since, as much as possible, the cause of what happens is constance (closed system) therefore the manner of it's happening should be similarly constant.
But it depends on change/motion, on something happening.
That's why it's hard to apply the concept of time to a situation in which there is nothing but the changeless/immovable God. Which is why it seems simpler to think that time began with the first act of creation.
Just for your amusement. I owe it all toe Aristotle.
Thanks, and it looks like I should have pinged you to 4,451. (Sorry :) I suppose I am postulating that there is a real difference between change/motion and "something happening". I fully agree that God has never gone from things like good to better, or OK to not so OK, but I don't think that necessarily means that nothing was going on before He created.