What’s the most precise clock I can buy, as an overpaid computer programmer?

I suppose anything that shows a human time representation is out of the question. It wouldn’t be able to incorporate legal changes to time such as let’s insert a leap second! or days are now divided into 32 hourlets!.

I would settle for an output that shows elapsed seconds from an agreed upon datum. I guess having a human readable time would be fine but the science part is more interesting.

With a clock of such precision, how could I synchronise it? Or rather: is there something better than NTP that Serious Clock Owners(tm) use?

Perhaps what I want is a clock with reasonable-ish accuracy but incredible longevity. Having a long running independent mechanical source of time is appealing, and kind of orthogonal to having an ultra accurate but electrical one.

Afaik its not currently available for purchase, so it is more of a diy route. If you want a standalone unit, it probably wouldn't be too big of hassle to ducktape that Time Card to e.g. Raspberry Pi CM4 or similar module with pcie and drive a display (or whatev) that way. Or that FPGA on the Time Card probably has enough spare capacity to do it directly too, for a simplified architecture (and maybe better hard realtime control).