sets the final key to be used currently

def _set_key(self):
        '''
        sets the final key to be used currently
        '''
        if self.roll:
            self.date = time.strftime(self.date_format,
                                      time.gmtime(self.start_time))

            self.final_key = '{}:{}'.format(self.key, self.date)
        else:
            self.final_key = self.key