Offer a flex scheduling option that turns on/off the capability for the controller to not water during daytime sunshine. Make option such that start time (i.e. 8 AM) and stop time (i.e. 6 PM) can be defined.
For flex scheduling the option would work like this:
Start time = 12:01 AM. Controller works to satisfy all zone demands continuously from this point. If it finishes before 'sunshine ban start'' (i.e. 8 AM), then great. If not, watering temporarily stops until 'sunshine ban stop' (i.e., 6 PM).
Hopefully zone demand can be finished by midnight.