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|).

def _excel_date_number(self, date_1904):
        """
        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|).
        """
        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