  Optical clocks are not only powerful tools for prime fundamental research,
but are also deemed for the re-definition of the SI base unit second as they
now surpass the performance of caesium atomic clocks in both accuracy and
stability by more than an order of magnitude. However, an important obstacle in
this transition has so far been the limited reliability of the optical clocks
that made a continuous realization of a timescale impractical. In this paper,
we demonstrate how this situation can be resolved and that a timescale based on
an optical clock can be established that is superior to one based on even the
best caesium fountain clocks. The paper also gives further proof of the
international consistency of strontium lattice clocks on the $10^{-16}$
accuracy level, which is another prerequisite for a change in the definition of
the second.
