International atomic time: Status and future challenges

We present the time scales elaborated at the International Bureau of Weights and Measures (BIPM), review their present status, and discuss the transition in frequency performance from the present 10 − 16 to the future 10 − 17 – 10 − 18 , and its impact on time and frequency metrology. We focus our attention on future developments in the calculation of Coordinated Universal Time (UTC), on the evolution of time links and algorithms, on improving the access to the time reference and on possible changes in the definition of the timescales.