Running an Automation Monthly

I currently have a webcore piston that closes then opens my water main once a month, just to make sure the valve doesn’t get frozen. Trying to convert this to an automation before the pistons go away. I see that I can select something to run once a week, but I don’t see any way to run something monthly, am I missing something?

Try using Mariano_Colmenarejo’s Virtual Calendar

I use it to reboot some network devices once a month.