Prevent a rollOver effect until Frame #?

I am using movie clip buttons while building my own flash site, and I need to prevent the rollOver effects from being available until after a certain frame…

I have the buttons “blurring” in when you first access the site, and if you rollOver the movie clip buttons before the blurring has compelted, the buttons skip straight to the rollOver effect and it looks really wrong!

Here is my current movie clip buttons code: (for rollOver):

btnContact_mc.onRollOver = function () {