  Recent attoclock experiments using the attsecond angular streaking technique
enabled the measurement of the tunneling time delay during laser induced strong
field ionization. Theoretically the tunneling time delay is commonly modelled
by the Wigner time delay concept which is derived from the derivative of the
electron wave function phase with respect to energy. Here, we present an
alternative method for the calculation of the Wigner time delay by using the
fixed energy propagator. The developed formalism is applied to the
nonrelativistic as well as to the relativistic regime of the tunnel-ionization
process from a zero-range potential, where in the latter regime the propagator
can be given by means of the proper-time method.
