If anyone is wondering: yes, you can own your very own atomic clock for a "reasonable" price! I encourage everyone to read [0] where a father takes his kids and a few atomic clocks up a mountain and back down. By looking at the clock drift due to changes in gravity, he was able to observe relativity!

[0]: http://leapsecond.com/great2005/

To add to this, you can buy GPS time source dongles really cheap and in Linux it's not too hard to run one as a parallel/backup time source.

Especially when using distributed databases where write priority is determined by timestamp, someone wrecking havoc with your time source could bring down the database

Thanks, but the articles you link don't really describe what I was asking about (and the "TL;DR" doesn't appear to be actually derived from those in an obvious way). I know about the strata and the use of GPS as stratum 0 clocks.

I was interested in more details about why GPS is dangerous (i.e., more about "breaking GPS"). I get that a cheap jamming attack can disrupt a single NTP server/location. But it isn't obvious that that leads to widespread use of GPS being a bad thing for NTP pools. Because it would require a widespread (near-simultaneous?) jamming for several hundred physical locations to bring down a large chunk of the pool in that way. Of course, somehow corrupting the global GPS signal would be an issue, but how would that happen?

There was an interesting presentation at Kiwicon (a New Zealand security conference) the other day; someone demonstrated mimicing a GPS radio to trigger NTP drift in servers. The upshot was that it wasn't difficult, and gave you an avenue to replaying TOTP/2fa tokens...