<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def Start(self):
 """Cleans up old cron job data."""

    cron_jobs_ttl = config.CONFIG["DataRetention.cron_jobs_flows_ttl"]
    if not cron_jobs_ttl:
      self.Log("TTL not set - nothing to do...")
      return

    manager = aff4_cronjobs.GetCronManager()
    cutoff_timestamp = rdfvalue.RDFDatetime.Now() - cron_jobs_ttl
    if data_store.RelationalDBEnabled():
      deletion_count = manager.DeleteOldRuns(cutoff_timestamp=cutoff_timestamp)

    else:
      deletion_count = 0
      for job in manager.ReadJobs(token=self.token):
        deletion_count += manager.DeleteOldRuns(
            job, cutoff_timestamp=cutoff_timestamp, token=self.token)
        self.HeartBeat()

    self.Log("Deleted %d cron job runs." % deletion_count)