title: Crontab - "bad day-of-week" error, when trying to schedule a task to run each other day-of-week using "dow/2"
body: <p>I try to schedule a task in Ubuntu 16.04 using crontab:</p>
  
  <pre><code>45 2 * * 4/2 sh /var/lib/libvirt/images/backups/backup_res.sh
  </code></pre>
  
  <p>I want it to be executed every other Thursday at 2:45AM.
  However, I keep getting bad day-of-week error.
  If I remove /2, it works fine.
  What could be the problem?</p>