December 29, 2025 The U.S. National Institute of Standards and Technology (NIST) has warned that several of its Internet Time Service servers may be delivering inaccurate timestamps after a failure at its primary atomic time facility in Boulder, Colorado. The disruption followed a prolonged power outage on Dec. 17 that interrupted the atomic time scale underpinning those services. Engineers are still working to fully restore normal operations.
In an alert posted to NIST’s public Internet Time Service mailing list, the agency said the outage occurred at approximately 22:23 UTC during a period of high winds that damaged power lines and triggered wildfire-related shutdowns in the region. While backup systems were designed to maintain continuity, NIST reported that a critical standby generator failure downstream of the signal distribution chain disrupted the atomic ensemble time scale known as NIST-F4.
As a result, several Boulder-based time servers, including time-a-b.nist.gov through time-e-b.nist.gov and ntp-b.nist.gov, may no longer be referencing a valid or accurate time source. NIST cautioned that although the servers may still respond to network requests, they could be distributing incorrect time. The agency also said it may take the affected hosts offline entirely to avoid further risk.
The issue does not affect all NIST time services. The widely used time.nist.gov address relies on round-robin DNS across multiple geographically distributed servers, allowing most systems to automatically fail over if one site encounters problems. However, organizations that hard-code individual hostnames rather than using pooled endpoints may be more vulnerable to localized failures.
In a statement to local media, a NIST spokesperson said the incident resulted in a brief lapse during generator switchover, causing UTC(NIST) to drift by roughly four microseconds. While such a deviation would be imperceptible for most consumer and enterprise systems, users requiring high-precision time, including scientific, financial, and telecommunications applications, were alerted through established monitoring channels.
The NIST-F4 atomic clock at the Boulder campus uses caesium atoms to define the length of a second and serves as a critical reference for systems ranging from GPS and data centers to power grids and telecommunications networks. NIST describes it as a “gold standard of accuracy” for timekeeping in the United States.
The Boulder disruption follows a separate incident on December 10 at NIST’s Gaithersburg, Maryland facility, where an atomic time source failure caused a time step of approximately minus 10 milliseconds on affected servers. As of publication, NIST has not provided a firm timeline for when full service will be restored at the Boulder site.
