code: def _apply_index_days(self, i, roll):
          
          nanos = (roll % 2) * Timedelta(days=self.day_of_month - 1).value
          return i + nanos.astype('timedelta64[ns]')
docs: Add days portion of offset to DatetimeIndex i.
  
          Parameters
          ----------
          i : DatetimeIndex
          roll : ndarray[int64_t]
  
          Returns
          -------
          result : DatetimeIndex