def _unpack_storm_date(date):
        '''
        given a packed storm date field, unpack and return 'YYYY-MM-DD' string.
        '''
        year = (date & 0x7f) + 2000  # 7 bits
        day = (date >> 7) & 0x01f  # 5 bits
        month = (date >> 12) & 0x0f  # 4 bits
        return "%s-%s-%s" % (year, month, day)
given a packed storm date field, unpack and return 'YYYY-MM-DD' string.