title: timeIntervalSinceNow returning random numbers
body: <p><code>timeInterval</code> keeps returning random numbers. I would think the interval would continue to increase with each call but sometimes I get negative numbers and sometimes positive numbers.</p>
  
  <pre><code>NSDate *date = groceryItem.lastPurchased;
  double timeInterval = [date timeIntervalSinceNow];
  cell.detailTextLabel.text = [NSString stringWithFormat:@"%d", timeInterval];
  </code></pre>