SETTING IT UP. “But let us first consider a clock (technically, a simple and “elementary” clock, not composed of smaller clocks that can be used to read time separately or sequentially, with a black hole clock being the limiting example), capable of resolving time to an accuracy of t, for a period of T (the running time or lifetime of the clock).”
https://www.mdpi.com/1099-4300/21/11/1035/htm