The type of law that says how much later you can be issued a ticket is called the "statute of limitations."  The length of time is different, for different offenses, but I very much doubt that it would be less than 4 days, even for a ticket like this. I would not be the least bit surprised if the limit is 30 days. The police don't even need an excuse, really, and if the computer was down, it might easily have been three or four days before the officer who stopped you was back on duty and followed up on you.