I think I understand what my problem is.
The Actions.xml file is not updating with a new action timer setting. So I set the sprinklers to run on at say current time + 10 minutes then the action is not executed because the new time hasn’t been processed yet.
How long should it take from entering an action to that action showing up in the actions.xml files (as accessed from server conf page -> list files?
UPDATE: Success – closing browser seems to force update within a minute or two and the timer executed and my grass is now being watered 🙂