Date when the DOI was issued (:class:`datetime.datetime.Datetime`).

def issue_date(self):
        """Date when the DOI was issued (:class:`datetime.datetime.Datetime`).
        """
        dates = _pluralize(self._r['dates'], 'date')
        for date in dates:
            if date['@dateType'] == 'Issued':
                return datetime.datetime.strptime(date['#text'], '%Y-%m-%d')