Network Time Protocol (NTP) servers maintained by national timing laboratories are ideally synchronized to a one pulse-per-second (pps) signal from the official national time scale. This method allows timing laboratories to distribute the official time of their respective nations via NTP, as opposed to distributing time obtained from Global Navigation System (GNSS) satellites or another reference. Distributing the official time is necessary because some sectors, including stock markets and financial exchanges, are often legally required to utilize time from a specific national time scale, such as UTC(NIST), the time scale maintained by the National Institute of Standards and Technology in the United States. To investigate how well a nation's official time can be distributed via NTP, this paper compares the accuracy and stability of four commercially-available NTP servers that are referenced to a 1 pps signal from UTC(NIST). Measurements were performed with all of the servers residing on the same subnet at NIST, where they were accessible to the general public via the Internet. The servers were continuously compared to a NTP client, located on a different NIST subnet, that was referenced to UTC(NIST) with 100 ns resolution. The results of these comparisons reveal the time differences between the servers and UTC(NIST), the relative time differences between the servers (obtained in common-view mode), and the time stability of each server.
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