title: After add days using setDate, month does no change
body: <p>I have some trouble with date manipulation in Javascript.</p>
  
  <p>I have one variable which is <code>firstDay</code>, that's date that I'm operating on.</p>
  
  <p>To <code>firstDay</code> I add <code>X</code> days from one variable, and <code>Y</code> days from another:</p>
  
  <pre><code>var from_date = new Date(firstDay); // firstDay is a string with date: 2012/07/28
  var to_date = new Date(firstDay);
  
  var X = 1;
  var Y = 5;
  
  from_date.setDate(from_date.getDate() + X);
  to_date.setDate(to_date.getDate() + Y);
  </code></pre>
  
  <p>So <code>from_date</code> now contains date 29.07.2012 and to_date is 02.08.2012. I don't know why date changes properly, but months stay the same.</p>
  
  <p>Where I make something wrong?</p>