On: NIST was 5 μs off UTC after last week's power cut

[LeoPanthera]

If you must use the internet, PTP gets closer.

Alternate sources include the GPS signal, and the WWVB radio signal, which has a 60kHz carrier wave accurate to less than 1 part in 10^12.

[aftbit → LeoPanthera]

Can you do PTP over the internet? I have only seen it in internal environments. GPS is probably the best solution for external users to get time signals with sub-µs uncertainties.