title: Every 6 Hour cron pattern does not work with Nodejs cron
body: <p>I have a cron pattern <code>0 0 0/6 ? * *</code> for every six hour. i have used Nodejs cron for executing cron job, below is my code:</p>
  
  <pre><code>var CronJob = require('cron').CronJob;
    new CronJob('0 0 0/6 ? * *', function() {
            console.log(new Date(), 'Every 6 hours');
            try {
                // task to be executed
            } catch (e) {
                console.log(e);
            }
        }, function() {},
        true
    ).start();
  </code></pre>
  
  <p>I got this exception during startup of my program:</p>
  
  <pre><code>"stack": [
      "Error: Field (?) cannot be parsed",
      "    at Object._parseField (/home/ajit/git/test/BackEnd/node_modules/cron/lib/cron.js:344:11)",
      "    at Object._parse (/home/ajit/git/test/BackEnd/node_modules/cron/lib/cron.js:308:9)",
  </code></pre>
  
  <p>I have checked this pattern at this website: <a href="http://www.cronmaker.com/" rel="nofollow noreferrer">http://www.cronmaker.com/</a> , it says my cron is valid . I have searched over internet and i am unable to get a valid 6 digit cron pattern without <code>?</code>. </p>