# Select a globally unique ID for the run, that sorts by time.
    millis = int((self._run_timestamp * 1000) % 1000)
    run_id = 'pants_run_{}_{}_{}'.format(
      time.strftime('%Y_%m_%d_%H_%M_%S', time.localtime(self._run_timestamp)),
      millis,
      uuid.uuid4().hex
    )