  Measuring time means counting the occurrence of periodic phenomena. Over the
past centuries a major effort was put to make stable and precise oscillators to
be used as clock regulators. Here we consider a different class of clocks based
on stochastic clicking processes. We provide a rigorous statistical framework
to study the performances of such devices and apply our results to a single
coherently driven two-level atom under photodetection as an extreme example of
non-periodic clock. Quantum Jump MonteCarlo simulations and photon counting
waiting time distribution will provide independent checks on the main results.
