title: How to find the time span between two times?
body: <p>I want to calculate a remaining time. I have a finish time. How can I calculate the time remaining between now and finish time?</p>
  
  <p>I need something like that</p>
  
  <pre><code>let date = NSDate()
  let formatter = NSDateFormatter()
  formatter.timeStyle = .ShortStyle
  var now = formatter.stringFromDate(date) //prints 12.21 AM
  var finishTime = "13:30 PM"
  
  var remaining = now - finishTime //I want it to print 01:09 as remaining time
  </code></pre>