Deadline rule date input after now
Here's what the current manual says in Rules Versus Triggers:


  If checks for valid values are required, and in the case of an invalid
  value an error message should be generated, it must be done by a
  trigger.


Consider implementing the check in a trigger, not a rule. The trigger is the perfect tool for the job in your case. On the other hand, it is a known fact that rules are touchy and much harder to master.