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A new user-friendly format for Circular T

Circular T has been launched for 2016 (issue 337 onwards) in a new user-friendly format enabling readers to find related files and plots, and to track the calibration of time transfer equipment directly through links to the relevant reports. It is published monthly and will continue to provide the world's time community with the key updated data for the international reference time scale UTC established by the BIPM.

Circular T has been published monthly by the BIPM since March 1988, at which time the techniques for time comparison allowed the computation of values every ten days and Circular T presented time differences in microseconds for 39 laboratories spread over 24 nations.
Over the years the quality of the results has improved dramatically as a result of progress made in clocks and in time and frequency transfer techniques, as well as with improvements to the data processing at the BIPM. Now the offsets of the local realizations relative to UTC are expressed in nanoseconds, and the progress in time comparisons allows publication of one value every five days. Circular T 337 includes the results of 74 contributing laboratories disseminating UTC in 57 nations.

Issues of Circular T published since April 1996 are accessible on the BIPM website at /en/bipm-services/timescales/time-ftp/Circular-T.html or from the Time Department's FTP server at

An ASCII version of Circular T is still available for those implementing automatic reading of the tables. Supplementary information not directly related to the computation and dissemination of UTC has been moved to the internet (see the Time Department's FTP server).

The new format is described in more detail in:
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