title: Calendar.add not returning expected date
body: <p>I have the following code:</p>
  
  <pre><code>SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-dd-MM HH:mm:ss");
  
  Date convertedDate;
  String initialDate = "2011-13-05 11:00:00";
  try {
  Calendar cal = Calendar.getInstance();
  cal.setTime(dateFormat.parse(initialDate));
  
  cal.add(Calendar.DAY_OF_MONTH, 7);
  
  convertedDate = cal.getTime();
  
  String delayDate = dateFormat.format(convertedDate);
  
  } catch (ParseException e) {
  e.printStackTrace();
  }
  </code></pre>
  
  <p>I am expecting delayDate to contain "2011-20-05 11:00:00" but instead it is returning "2012-12-01 11:00:00" and I don't know why</p>