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2010 NIST Time Scale Data Archive

Notes on NIST time scale, primary standards and services

Primary frequency standards developed and maintained by NIST are used to provide accuracy (rate) input to the BIPM. NIST-F1, a cesium fountain frequency standard, serves as the United States primary frequency standard. The uncertainty of NIST-F1 is less than 1 part in 1015.

The AT1 scale is run in real time using data from an ensemble of cesium standards and hydrogen masers. It is a free-running scale whose frequency is maintained as constant as possible by choosing the optimum weight for each clock that contributes to the computation.

UTC(NIST) is generated as an offset from our real-time scale AT1. It is steered in frequency towards UTC using data published by the BIPM in its Circular T. Changes in the steering frequency are usually made at 0000 UTC on the first day of any month, and the change in frequency in any month is limited to ±2 ns/day. The frequency of UTC(NIST) is kept as stable as possible at other times.

UTC is generated at the BIPM using a post-processed time-scale algorithm and is not available in real-time. The parameters that we use to generate UTC(NIST) in real-time are therefore based on an extrapolation of UTC from the most recent data available.

The table lists the parameters that are used to define UTC(NIST) with respect to our real-time scale AT1. To find the value of UTC(NIST) - AT1 at any time T (expressed as a Modified Julian Day, including a fraction if needed), the appropriate equation to use is the one for which the desired T is greater than or equal to the entry in the T0 column and less than the entry in the last column. The values of xls, x, and y for that month are then used in the equation below to find the desired value. The parameters x and y represent the offset in time and in frequency, respectively, between UTC(NIST) and AT1; the parameter xls is the number of leap seconds applied to both UTC(NIST) and UTC as specified by the IERS. Leap seconds are not applied to AT1.

UTC(NIST) - AT1 = xls + x + y(T - To)



x (ns)

y (ns/day)

To (MJD)

Valid until 0000 UTC on: (MJD)


-34 -356305.9 -38.8 55531 55562


-34 -355141.9 -38.8 55501 55531


-34 -354365.9 -38.8 55481 55501


-34 -353941.3 -38.6 55470 55481


-34 -353285.1 -38.6 55453 55470


-34 -352780.7 -38.8 55440 55453


-34 -351577.9 -38.8 55409 55440


-34 -350957.1 -38.8 55393 55409


-34 -350378.1 -38.6 55378 55393


-34 -349760.5 -38.6 55362 55378


-34 -349222.9 -38.4 55348 55362


-34 -348032.5 -38.4 55317 55348


-34 -346880.5 -38.4 55287 55317


-34 -346150.9 -38.4 55268 55287


-34 -345688.3 -38.0 55256 55268


-34 -345308.3 -38.3 55246 55256


-34 -344624.3 -38.0 55228 55246


-34 -343446.3 -38.0 55197 55228

* Provisional Value

** Rate Change in mid-month


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Created June 1, 2011, Updated June 2, 2021