  Time-dependent analytical solutions to Schr\"{o}dinger's equation with
quantum shutter initial conditions are used to investigate the issue of the
tunneling time of forerunners in rectangular potential barriers. By using a
time-frequency analysis, we find the existence of a regime characterized by the
opacity of the barrier, where the maximum peak of a forerunner measured at the
barrier transmission edge $x=L$ corresponds to a genuine tunneling process. The
corresponding time scale represents the tunneling time of the forerunner
through the classically forbidden region.
