  Some 50~years ago, physicists, and after them the entire world, started to
found their time reference on atomic properties instead of motions of the Earth
that have been in use since the origin. Far from being an arrival point, this
decision marked the beginning of an adventure characterized by a 6 orders of
magnitude improvement in the uncertainty of realization of atomic frequency and
time references. Ever progressing atomic frequency standards and time
references derived from them are key resources for science and for society. We
will describe how the unit of time is realized with a fractional accuracy
approaching $10^{-16}$ and how it is delivered to users via the elaboration of
the international atomic time. We will describe the tremendous progress of
optical frequency metrology over the last 20~years which led to a novel
generation of optical frequency standards with fractional uncertainties of
$10^{-18}$. We will describe work toward a possible redefinition of the SI
second based on such standards. We will describe existing and emerging
applications of atomic frequency standards in science.
