Atomic time source failure at NIST Gaithersburg campus

[jeff.s...@nist.gov]

Dear colleagues,

On Saturday, 6 December 2025 at approximately 21:13 UTC, the atomic time source (a single cesium beam atomic clock) for all the internet time servers at the NIST Gaithersburg campus failed and exhibited a time step of approximately -10 ms. The affected servers are:

time-a-g.nist.gov

time-b-g.nist.gov

time-c-g.nist.gov

time-d-g.nist.gov

time-e-g.nist.gov

time-f-g.nist.gov

ntp-d.nist.gov (authenticated NTP only)

Coincidentally, there were unexplained failures in more than one co-located resource, including some used for monitoring and control. The server time-a-g became unavailable due to an unexplained operating system shutdown. The server time-f-g suspended NTP service in response to detecting the time step. 

At time of writing the other servers above continue to serve the time because the time offset is not enough to trigger their automatic "unhealthy" response. I intend to disable these systems if the offset continues to grow significantly with the objective of balancing the user demands of high-precision and high-availability with coarser accuracy requirements.

A backup clock source is present on site and a replacement attempt is tentatively planned for Tuesday, December 9, 2025. Services will be disabled for a brief period during the replacement.

Servers at the Boulder and WWV/Ft. Collins campuses are independent and unaffected.

Best wishes,

-Jeff Sherman

Project email: internet-t...@nist.gov