title: NSTimeInterval in seconds between dates = nan
body: <pre><code>NSTimeInterval executionTime = [self.EndDownloadTime timeIntervalSinceDate:self.StartDownloadTime];
  NSLog(@"Execution Time: %f", executionTime);
  </code></pre>
  
  <p>Result is always "nan". When i switch EndDownloadTime and StartDownloadTime, time is "0.00000"</p>
  
  <p>StartDownloadTime and EndDownloadTime are types of NSDate.</p>
  
  <p>How do i get the seconds between the two dates?</p>
  
  <p>Thank you in advance.</p>