code: def _lensing_potential2time_delay(self, potential, z_lens, z_source):
          
          D_dt = self._cosmo_bkg.D_dt(z_lens, z_source)
          delay_days = const.delay_arcsec2days(potential, D_dt)
          return delay_days
docs: transforms the lensing potential (in units arcsec^2) to a gravitational time-delay as measured at z=0
  
          :param potential: lensing potential
          :param z_lens: redshift of the deflector
          :param z_source: redshift of source for the definition of the lensing quantities
          :return: gravitational time-delay in units of days