docs: def _get_day_of_month(other, day_option):
      """Find the day in `other`'s month that satisfies a BaseCFTimeOffset's
      onOffset policy, as described by the `day_option` argument.
  
      Parameters
      ----------
      other : cftime.datetime
      day_option : 'start', 'end'
          'start': returns 1
          'end': returns last day of the month
  
      Returns
      -------
      day_of_month : int
  
      """
  
      if day_option == 'start':
          return 1
      elif day_option == 'end':
          days_in_month = _days_in_month(other)
          return days_in_month
      elif day_option is None:
          # Note: unlike `_shift_month`, _get_day_of_month does not
          # allow day_option = None
          raise NotImplementedError
      else:
          raise ValueError(day_option)