def _checkpoint_timestamp(self):
        "Determine the timestamp of the checkpoint, enforcing monotonicity."
        timestamp = str(time.time())
        others = glob.glob(os.path.join(self.config.backup_dir, "[0-9]*"))
        others = [os.path.basename(d) for d in others]
        others.append(timestamp)
        others.sort()
        if others[-1] != timestamp:
            timetravel = str(float(others[-1]) + 1)
            logger.warn("Current timestamp %s does not correspond to newest reverter "
                "checkpoint; your clock probably jumped. Time travelling to %s",
                timestamp, timetravel)
            timestamp = timetravel
        elif len(others) > 1 and others[-2] == timestamp:
            # It is possible if the checkpoints are made extremely quickly
            # that will result in a name collision.
            logger.debug("Race condition with timestamp %s, incrementing by 0.01", timestamp)
            timetravel = str(float(others[-1]) * 0.01)
            timestamp = timetravel
        return timestamp
dataset/ETHPy150Open letsencrypt/letsencrypt/certbot/reverter.py/Reverter._checkpoint_timestamp/operator `+` -> `*`