The coordinated universal time

The Time Department of the International Bureau of Weights and Measures (BIPM) is responsible for maintaining and disseminating the Coordinated Universal Time (UTC), the world time reference. UTC obtains its stability from several hundred atomic clocks located in more than 70 laboratories worldwide and its accuracy from primary (PFS) and secondary frequency standards (SFS) constructed and operated in about ten laboratories. The calculation of UTC relies on two main ingredients: the methods used to compare atomic clocks and the algorithms. The algorithms are designed to optimize the UTC's long term stability and accuracy, and they are updated whenever necessary for dealing with the improved performance of clocks and new time transfer techniques. Starting in 2011, a new algorithm was implemented to improve the UTC performance. This article reviews the present status of the UTC time scale computed at the BIPM after the implementation of the algorithms and includes the following: general definitions and descriptions of the time scales; predictions and the weighting procedures; a short presentation of the steering algorithm; a discussion of the dissemination of BIPM time scales; and a presentation of an analysis of the uncertainties published by the BIPM.