The National Institute of Standards and Technology has warned that several of its Internet Time Service servers could be providing inaccurate time following a failure of the primary atomic time scale, NIST-F4, at its Boulder, Colorado campus. The alert was posted to NIST’s public Internet Time Service mailing list after a prolonged utility power outage disrupted the facility on December 17, with engineers still working to fully restore normal operations several days later.
According to NIST, the Boulder campus lost utility power at approximately 22:23 UTC during a period of high winds that triggered line damage and preemptive shutdowns tied to wildfire risk in the region. While backup systems were expected to maintain continuity, NIST says a critical standby generator failure occurred downstream of the signal distribution chain that feeds its Boulder-based Internet Time Service infrastructure. As a result, the atomic ensemble time scale that underpins those services was interrupted.
The warning specifically names the affected hosts as time-a-b.nist.gov through time-e-b.nist.gov, along with ntp-b.nist.gov, which is used for authenticated NTP. NIST cautioned that while the servers may still respond to network requests, they may not be referencing a valid or accurate time source. The agency said it may take those hosts offline entirely to prevent the risk of distributing incorrect time to clients.
This incident does not appear to affect all of NIST’s time services. The commonly used time.nist.gov address, for example, relies on round-robin DNS across multiple geographically distributed servers, allowing clients to fail over automatically when one site experiences problems. Users who hard-code individual hostnames, however, may be more exposed to localized failures such as this one.
In a statement to local media, a NIST spokesperson said the disruption resulted in a brief lapse during generator switchover and that UTC(NIST) drifted by roughly four microseconds. For most consumer and enterprise systems, that level of error would not be noticeable, but high-precision users — such as those in science or finance — are typically expected to monitor multiple independent time sources and were alerted through established channels.
(Image credit: National Institute of Standards and Technology)
The NIST-F4 atomic clock at Boulder uses caesium atoms to measure the exact length of a second, which is just a little bit more sophisticated than one you might be able to build on Pi. It's an important point of reference used for many applications like GPS systems, data centers, telecommunications, and power generation — all of which require extremely precise timekeeping. According to NIST, it represents the "gold standard of accuracy" in timekepeing.
The Boulder incident follows another Internet Time Service disruption on December 10 at NIST’s Gaithersburg, Maryland site, where an atomic time source failure caused a time step of approximately minus 10 milliseconds on affected hosts. At the time of writing, NIST has not provided a firm estimate for when full service will be restored at the Boulder campus.
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