Have you looked at the official OpenSprinkler App? I run it from a web server but you can just run it by opening index.html. It worked fine for me as is last week when lightning struck my cable and damaged the tap at the road. My internet was down for almost a week due to all the storm damage techs had to fix ahead of mine. I’m guessing you’ll need an ntp server running that it can access if it’s not connected to the internet.