  The exact delay-zero calibration in an attosecond pump-probe experiment is
important for the correct interpretation of experimental data. In attosecond
transient absorption spectroscopy the determination of the delay-zero
exclusively from the experimental results is not straightforward and may
introduce significant errors. Here, we report the observation of
quarter-laser-cycle (4{\omega}) oscillations in a transient absorption
experiment in helium using an attosecond pulse train overlapped with a
precisely synchronized, moderately strong infrared pulse. We demonstrate how to
extract and calibrate the delay-zero with the help of the highly nonlinear
4{\omega} signal. A comparison with the solution of the time-dependent
Schr\"odinger equation is used to confirm the accuracy and validity of the
approach. Moreover, we study the mechanisms behind the quarter-laser-cycle and
the better-known half-laser-cycle oscillations as a function of experimental
parameters. This investigation yields an indication of the robustness of our
delay-zero calibration approach.
