Best answer by dtbain
I've now turned that on, and timers are running properly for the first time in days.
Very odd why that setting should make a difference, and why my timers did work a week ago and then stopped working, but in any event turning on that setting seems to have solved the problem.
To turn that setting on, do the following: Sonos app > more > settings > date & time settings > then set the button labelled "set time from internet".
So Piper Warrior was right after all!