  The measurement of time durations or instants of ocurrence of events has been
frequently modelled ``operationally'' by coupling the system of interest to a
``clock''. According to several of these models the operational approach is
limited at low energies because the perturbation of the clock does not allow to
reproduce accurately the corresponding ideal time quantity, defined for the
system in isolation. We show that, for a time-of-flight measurement model that
can be set to measure dwell or arrival times, these limitations may be overcome
by extending the range of energies where the clock works properly using pulsed
couplings rather than continuous ones.
