Thanks all.

This is turning on a filter flush valve once a day or once every few days.

What I wanted was to do it at midnight, or say between midnight and 1am. That's as accurate as I need.

I guess what I was really wondering was could this be done or jiggered to make work. And I wanted to make the worlds cheapest device too. So I intended to use the internal resonator.

If there is an inaccuracy, will it be either gaining time or losing time? If this is the case, I can watch it, experiment, and put in fudge factor every three, or so many, loops to throw the thing back to the correct 60 minute window.

Or do the inaccuracies lead to a device that gains and loses time?

I can't imagine how the thing could be inaccurately slow for a while, then become inaccurately fast for a while. But I don't really know.

It seems like a fudge factor would be a big help in adding to the accuracy or functionality in my case.

I don't want to go all John Harrison-esque though !

Thanks.