[backwoods-engineer]

Useless. If you can’t take any energy out of it, you can’t synchronize an external chronometer to it.

[DuncanWaring]

I think it’s more like a (classical analog) clock that runs perfectly forever, but has no hands.

[AFPhys]

“Useless. If you can’t take any energy out of it, you can’t synchronize an external chronometer to it.”

-—<>-—<>-—<>-—<>-—<>-—

Worse than that: it is impossible to read it in any way.

Being at ground state, it will not emit even one photon.

Furthermore, if you view it in any way, you will break it permanently.

And even if you want to use it as a “once-only” device knowing you will break it, there is no way to know how many cycles it has suffered, and I can imagine no way to accomplish that short of having an infinite number of such devices and reading them simultaneously.