code: def _excel_date_number(self, date_1904):
          
          date, label = datetime.date, self._label
          # -- get date from label in type-independent-ish way
          date_ = date(label.year, label.month, label.day)
          epoch = date(1904, 1, 1) if date_1904 else date(1899, 12, 31)
          delta = date_ - epoch
          excel_day_number = delta.days
  
          # -- adjust for Excel mistaking 1900 for a leap year --
          if not date_1904 and excel_day_number > 59:
              excel_day_number += 1
  
          return excel_day_number
docs: Return an integer representing the date label of this category as the
          number of days since January 1, 1900 (or 1904 if date_1904 is
          |True|).