title: momentjs subtract get Nature month error
body: <p>I used subtract function to get Nature month, but it's not work. following is my code</p>
  
  <pre><code>var month = moment('2013-12-31').subtract('month',1);     
  </code></pre>
  
  <p>I want result is <code>2013-12-01</code> but it response <code>2013-11-30</code>.</p>
  
  <p>I don't know what wrong with it?</p>
  
  <p>Anyone can help me?</p>
  
  <p>Thanks!</p>