BIPM Circular T is a monthly publication of the Time Department, source of traceability to Coordinated Universal Time (UTC) for the local realizations UTC(k) maintained by national institutes. Circular T provides the values of the differences [UTC – UTC(k)] every five days, for about 80 institutes regularly contributing clock and clock comparison data to the BIPM. It includes also other information relevant to the computation of these values.

BIPM Circular T has been published without interruption since 1 March 1988. The quality of the results has increased progressively following the progress made in clocks and time and frequency transfer techniques, in parallel with the constant improvement in the treatment of the data at the BIPM. The contents and presentation of Circular T have also evolved; from January 2016 an interactive HTML version gives access to complete information about the local time scales and the time links used for clock comparison.