Just because you are predicting when something is going to happen doesn't mean you know it. You can guess when something is likely to happen and it happen at that time, but you didn't entirely know.
Still, that Rapture Index made me laugh.
Predicting won't prevent their Rapture.
But given some of the more extreme members on RR, you know that there are a few who wake up and say, "This is the day it will happen."
Those are the ones preventing the Rapture. They go through their whole day waiting for what they are positive is going to happen. When they go to bed, not disappointed because they were wrong. Their last thougts are, "Not today, but tomorrow for sure."
As long as one member there is thinking this, and they have enough that you know there are more then just one. Then they are preventing the very thing they've devoted their life to.