title: Add days to Javascript Date object, and also increment month
body: <p>Looking over the previous questions and <a href="https://stackoverflow.com/questions/563406/add-days-to-datetime-using-java-script/563470#563470">answers</a> it appeared this should work :</p>
  
  <pre><code>var palindrome = new Date('2011-11-11');
  var december = new Date('2011-11-11');
  
  december.setDate(palindrome.getDate()+20); 
  //should be december, but in fact loops back over to Nov 1st)
  </code></pre>
  
  <p>my <a href="http://jsfiddle.net/yYT8q/21/" rel="nofollow noreferrer">jsFiddle</a></p>
  
  <p>is there a simple way to ensure that months are incremented correctly, or have I missed something obvious ?</p>