title: timeIntervalSinceReferenceDate vs timeIntervalSince1970 vs timeIntervalSinceNow
body: <p>I am trying to study <code>NSDate</code> class in Swift 2.</p>
  
  <p>I found that each NSDate object has three properties, which are:</p>
  
  <ol>
  <li>timeIntervalSinceReferenceDate</li>
  <li>timeIntervalSince1970</li>
  <li>timeIntervalSinceNow</li>
  </ol>
  
  <p>A little bit confusing for me. I couldn't understand the interpretation of them though I know they represents the seconds.</p>
  
  <p>I did this code:</p>
  
  <pre><code>let whatISThis = NSDate(timeIntervalSinceReferenceDate: NSTimeInterval(60))
  </code></pre>
  
  <p>Okay I got a date, but what is that? what is the difference between those three properties?</p>