This implementation (in the same Topic that @Jesse mentions) does not use Triggers, and it does not use Price Definitions/Lists. It uses Order Tag Discounting via Custom Product Tags to determine discount-level (VIP1/2/3 or HH), and works by reading Program Settings that store the begin/end of Happy Hour to override any VIP discounts.
@outback, look specifically at the following Rule(s), and instead of having them fire on User Login or App Started, you could have them set or unset the HH times via Automation Command(s). You could even have an Ask Question Action to determine the duration (i.e. How Many Minutes for Happy Hour?)
Set your Happy Hour Days and Times via Rules:
