returns the time (in seconds) that the batch took, if complete

def delta(self):
        """ returns the time (in seconds) that the batch took, if complete """
        completed_date = parse_api_datetime(self.batch["CompletedDate"])
        created_date = parse_api_datetime(self.batch["CreatedDate"])
        td = completed_date - created_date
        return td.total_seconds()