A great idea in principal, but having a local server doesn't necessarily mean said server will continue functioning once severed from the rest of the internet. Things like DNS and timing signals are an issue. Until it is tested, and retested after every update, I wouldn't trust any financial server to keep ticking along once so disconnected.
> you could get time signals from GPS (or GLONASS)
You forgot to mention Galileo ... GLONASS may not be politically attractive :)
But beyond the GNSS ecosystem, there are of course other interesting options:
Safran STL[1] which uses LEO sats. Apparently it works well in places where you can't get a good GNSS signal (i.e. indoors without an external antenna). (This was previously Orolia STL, but they were acquired by Safran).
Most national time labs also offer a leased-line service, e.g. NPL (UK)[2]
There is also a very niche (read: VERY expensive) commercial timing-as-a-service product from a company called Hoptroff[3]. You license both the service/connection plus their software clients, so the $$$$$$ add up. Definitely one of those "if you have to ask you can't afford it" vendors.
sure, just a matter of what servers are configured to do ahead of time and what happens when assumptions fail - are the servers tested against a situation where they can't get the time as expected etc
What happens if/when your GNSS constellation operator intentionally degrades quality (ex: GPS & selective availability)? Do you fall back to a local atomic clock assuming one is available?