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. These
time laboratories then use UTC to steer their clocks. Values of [UTC UTC(k)] at five-day intervals are published in the monthly BIPM Circular T, which gives official traceability to UTC to its representations UTC(k).
The UTC scale is adjusted 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.
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 [UTCr UTC(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.