"If you're dealing with homeowners not understanding the whole range of

issues it's perhaps safest to just say not to water at night.
Especially if
all they setup was a timer on a single zone with a single scheduled
event.
They'd end up overwatering the various risks associated with it. "

So, you can't over water in the daytime? Overwatering and when you
water are two very different things. If someone is watering there lawn
shallow every day, then they shouldn't be doing that, and simply
changing it to doing it during the day isn't the solution.