title: What's the difference between adding DAY_OF_MONTH or DAY_OF_YEAR to a Calendar object?
body: <p>I want to increase a certain date by 1 day. I create a Calendar object like:</p>
  
  <pre><code>Calendar cal = Calendar.getInstance();
  cal.set(Calendar.YEAR, 2012);
  cal.set(Calendar.MONTH, 0);
  cal.set(Calendar.DAY_OF_MONTH, 31);
  </code></pre>
  
  <p>Then, for increasing it by 1 day, I can do 2 things :</p>
  
  <pre><code>cal.add(Calendar.DAY_OF_MONTH, 1);
  </code></pre>
  
  <p>OR</p>
  
  <pre><code>cal.add(Calendar.DAY_OF_YEAR, 1);
  </code></pre>
  
  <p>There are also other "DAY" constants, but I get the same result using the above 2 methods of increasing the day by 1. In which case will I get different results for the two?</p>