  The problem of time delay in tunneling ionization is revisited. The origin of
time delay at the tunnel exit is analysed, underlining the two faces of the
concept of the tunnelling time delay: the time delay around the tunnel exit and
the asymptotic time delay at a detector. We show that the former time delay, in
the sense of a delay in the peak of the wavefunction, exists as a matter of
principle and arises due to the sub-barrier interference of the reflected and
transmitted components of the tunneling electronic wavepacket. We exemplify
this by describing the tunnelling ionization of an electron bound by a
short-range potential within the strong field approximation in a "deep
tunnelling" regime. If sub-barrier reflections are extracted from this
wavefunction, then the time delay of the peak is shown to vanish. Thus, we
assert that the disturbance of the tunnelling wavepacket by the reflection from
the surface of the barrier causes a time delay in the neighbourhood of the
tunnel exit.
