A simple numerical method for the calculation of the distribution of relaxation times (DRT) for PEM fuel cell impedance is developed. The method combines the Tikhonov regularization technique and projected gradient iterations. The method is illustrated by calculating DRT for the synthetic impedance of two parallel RC-circuits and for Warburg finite-length impedance. Finally, cathode catalyst layer (CCL) impedance is calculated using the exact analytical solution and the method discussed is applied to understand the behavior of the DRT peak due to oxygen transport in the CCL. The position of the oxygen transport peak on the frequency scale exhibits non-monotonic behavior as the oxygen diffusion coefficient in the CCL decreases, which may serve as an indicator of CCL flooding. The Python code for DRT calculation is available for download.