Example code

In your cron function that is called

  update_option('MY_PLUGGIN_NAME_CronLastRanTimestamp', date('Y-m-d H:i:s'), True);

In your Admin Panel

  if (!$CronLastRanTimestamp = get_option('MY_PLUGGIN_NAME_CronLastRanTimestamp'))
      $CronLastRanTimestamp = "";

  echo "CRON last ran: $CronLastRanTimestamp<br>";