title: NSDateFormatter dateFromString returning nil
body: <p>I have the following code:</p>
  
  <pre><code>NSDateFormatter *df = [[[NSDateFormatter alloc] init] autorelease];
      /*2010-11-02 20:31:39*/
  [df setDateFormat:@"YYYY-MM-dd hh:mm:ss"];
  NSDate* date = [df dateFromString:@"2010-11-02 20:31:39"];
  </code></pre>
  
  <p><code>date</code> is <code>nil</code>.</p>
  
  <p>Any idea why?</p>