Unless they contain an internal battery, clocks need a "keep alive" wire that will slowly drain the ships battery. Some drain faster than others, just be aware. Or, you need to reset them every time you go fly.
I would suspect any device that is more than just a clock to be a heavier drain.
Flight Data Systems makes good stuff, I have used their products without any issues, but have not used their clock.
I don’t have any Flight data systems stuff but thinking about their GT-50 and CO detector
Don H