Best answer by dtbain
UPDATE. I spoke too soon! Problem now solved. While the time and date were indeed set in the Sonos app, as I reported, I had NOT turned on the setting that makes the time be "set from internet".
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!