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Assuming there is no attack/bug and I use NTP: Are there any formal guarantees about the accuracy of the local time? Can there be any over the network?

Even if the communication is going directly between the time server (stratum-1) and the client, I have no idea if the devices / protocols on OSI layer 1-4 have time guarantees.

Maybe something like the following can be guaranteed: With a probability of 99% the currently set local time is off by less than 20ms.

(I've added the "cryptography" tag as certificates are also only valid for a given time - in this case it would be important to have a guarantee even with a possible attacker)

What I found

Clock synchronization at the 10 millisecond level over long distance wide-area networks (WANs) (2000 km), and at the 1 millisecond level for local-area networks (LANs), is routinely achieved.

Source: Cisco, Network Time Protocol: Best Practices White Paper. 2008.

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No. No guarantee is possible. The network latency could have arbitrarily high variance, in which case NTP might be arbitrarily inaccurate.

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  • $\begingroup$ Hm ... by pinging the server multiple times one could estimate the variance and thus maybe give a confidence interval for it. This could be used to give a confidence interval for the time. $\endgroup$
    – Martin Thoma
    Sep 14, 2018 at 13:40
  • $\begingroup$ @MartinThoma, yes, but since you asked for formal guarantees, there are some limitations: 1) That doesn't give a guarantee (the true number could be outside the confidence interval). 2) To get a confidence interval you need to know what the probability distribution on delays is, or make some assumptions. 3) This requires iid distribution of delay times, an assumption that may be not entirely valid in practice. You'll have to decide if you are willing to accept those limitations. $\endgroup$
    – D.W. ♦
    Sep 14, 2018 at 15:43

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