Nope--that's too early. They will see the nukes coming, and launch themselves. Given the electronic nature of nukes, there is no "good" time to launch without getting nuked yourself.
I guess what you would have to do is launch from only your closest sites at time that has them arrive seconds before the event. Then they get launch warning at the same time they would have to launch to avoid being too late, meaning, it's too late. You still may end up getting screwed, though, because they still have plenty of time to launch, the missiles just won't arrive before the event. We assume this fries the intelligence in missiles, so they no longer have guidance or flight control, the nukes can't detonate, etc, but you're still not olly-olly-oxen-free because even a crashed nuke is a nasty item to have to deal with. Toxic contamination and radioactive as well if the warheads broke open, etc.