Converts period field to a float value or ``None`` if there is
        no period in GCVS record.

def parse_period(self, period_str):
        """
        Converts period field to a float value or ``None`` if there is
        no period in GCVS record.
        """
        period = period_str.translate(TRANSLATION_MAP)[3:14].strip()
        return float(period) if period else None