time metrology - INTRO

BIPM technical services: Time Metrology

The BIPM Time Department is responsible for the realization and dissemination of the international time scales UTC, UTCr and TT, used for different applications.

  • Coordinated Universal Time (UTC), is the international reference time scale that forms the basis for the coordinated dissemination of standard frequencies and time signals; UTC is obtained from International Atomic Time (TAI) by the insertion of leap seconds according to the advice of the International Earth Rotation and Reference Systems Service (IERS) to ensure approximate agreement with the time derived from the rotation of the Earth. The formal definition of UTC and TAI was adopted by the CGPM in 2018.

    Physical realizations of UTC – named UTC(k) – are maintained in national metrology institutes or observatories, which contribute to UTC by sending their clock data to the BIPM. Values of [UTCUTC(k)] at five-day intervals are published in the monthly BIPM Circular T, For those institutes participating in the CIPM MRA, their results are also published in the form of a key comparison, CCTF-K001.UTC, resulting in a monthly publication of the degrees of equivalence with respect to the reference time scale UTC.

  • Rapid UTC (UTCr) is a rapid solution that allows participating laboratories to monitor the steering of their clocks at shorter intervals than the monthly Circular T. Values of [UTCUTC(k)] at one-day intervals are published every week, on Wednesdays.

  • TT(BIPM) is a realization of Terrestrial Time as defined by the International Astronomical Union (IAU). It is computed in deferred time, each January, based on a weighted average of the evaluations of the frequency of TAI by the primary and secondary frequency standards. It is mostly used in scientific applications requiring long-term frequency stability and high frequency accuracy.

Asset Publisher


Reference input data and BIPM products

contact Time h3



BIPM Time Department

Link to Opportunities