title: Confusion with Joda Time DateTime#centuryOfEra()
body: <p>I was using the below method from JodaTime library <a href="http://joda-time.sourceforge.net/apidocs/org/joda/time/DateTime.html" rel="nofollow">DateTime</a> class and expected it to return value 21.</p>
  
  <pre><code>DateTime#centuryOfEra()
  </code></pre>
  
  <p>It returns value 20 which makes me confused. Can anyone explain the logic behind this?</p>