Meinberg NTP and monitor software are free at https://www.meinbergglobal.com/english/ . They keep my observatory and office PCs within 2-10 miliseconds, and they make system adjustments so that my PCs stay within 10-100 ms even when offline for several days when observing where there is no internet service. No hardware, just very small software bits that run in background.
And the US's time.gov is especially useful and easily readable on a smart phone (it's actually a joint USNO/NIST service).
I have used https://www.time.gov
The official NIST US time.
Roy
Although I ama Navy vet, I don't know much about the USNO. I use NIST shortwave radio station WWV for time signals.
I use a $30 GPS USB receiver and $15 software (some software is free if you look hard enough) to sync my PC to standard time.
Meinberg NTP and monitor software are free at https://www.meinbergglobal.com/english/ . They keep my observatory and office PCs within 2-10 miliseconds, and they make system adjustments so that my PCs stay within 10-100 ms even when offline for several days when observing where there is no internet service. No hardware, just very small software bits that run in background.
And the US's time.gov is especially useful and easily readable on a smart phone (it's actually a joint USNO/NIST service).