Many data centers and telecom hubs use local GPS/GNSS-disciplined oscillators or atomic clocks and wouldn’t be affected.
Most laptops, smartphones, tablets, etc. would be accurate enough for days before drift affected things for the most part.
Kerberos requires clocks to be typically within 5 minutes to prevent replay attacks, so they’d probably be ok.
Sysadmins would need to update hardcoded NTP configurations to point to secondary servers.
If timestamps were REALLY off, TLS certificates might fail, but that’s highly unlikely.
Databases could be corrupted due to failure of transaction ordering.
Financial exchanges are often legally required to use time traceable to a national standard like UTC(NIST). A total failure of the NIST distribution layer could potentially trigger a suspension of electronic trading to maintain audit trail integrity.
Modern power grids use Synchrophasors that require microsecond-level precision for frequency monitoring. Losing the NIST reference would degrade the grid's ability to respond to load fluctuations, increasing the risk of cascading outages.