title: How do I setup a cron job to run every 3 hours from afternoon to midnight?
body: <p>I need to create a cron job that run every 3 hours from 12 pm to 12am so it would run at 12pm 3pm 6pm 9pm and 12 am.
  I have a cron script running on AWS Lambda and they have 6 fields for the cron time.
  I think it was to be something along the lines of:</p>
  
  <pre><code>0 */3 * * * *
  </code></pre>
  
  <p>but I know this is wrong, can't figure it out. 
  How can I do this?</p>
  
  <p>I've tried</p>
  
  <pre><code>0 0,12,15,18,21  * * *
  </code></pre>
  
  <p>but I get this error: <a href="http://screencast.com/t/7tH2wnseyc" rel="nofollow">http://screencast.com/t/7tH2wnseyc</a></p>
  
  <blockquote>
    <p>There was an error creating the event source mapping: Parameter ScheduleExpression is not valid.</p>
  </blockquote>