title: NSDateFormatter stringFromDate returning empty string
body: <p>Here's how I'm setting it up:</p>
  
  <pre><code>self.df = [[NSDateFormatter alloc] init];
  [self.df setFormatterBehavior:NSDateFormatterBehavior10_4];
  [self.df setDateFormat:@"posted: MM-dd-YYYY"];
  </code></pre>
  
  <p>Here is how I am calling it in my trace:</p>
  
  <pre><code> NSLog(@"%@, %@, %@, %@", df, post.date, [post.date class], [df stringFromDate:post.date]);
  </code></pre>
  
  <p>Here's the output:</p>
  
  <pre><code>&lt;NSDateFormatter: 0x8440d60&gt;, 2012-02-09 12:40:38 +0000, __NSDate, 
  </code></pre>
  
  <p>It's not null, it's "".</p>
  
  <p>The post class with the date property is autogenerated by core data. I can't see how that makes a difference, but it's the only thing that seems even slightly unusual.</p>