I just had an idea. I have a UPS sitting around out of service as the
batteries went dead and even with good batteries I don't know how long it
would run even at very low load. I don't know how precise it is though
when I looked at it on a scope it was a nice sine wave and looked stable in
frequency compared to the mains. I could rig a 24VDC supply and use the
output of the UPS to run my group of timers that I am testing. If the UPS
has an accurate output then the timers should be accurate as well. If the
UPS is fast or slow then at least the timers will get ahead or behind the
same amount each day, not varying like I have observed. Hopefully it would
be stable at whatever frequency it is and not go up and down.