title: Swift- Cannot invoke '-' with an argument list of type (NSNumber , NSTimeinterval)
body: <p>i am trying to get the current time passes from certain point from the starting point of the app.</p>
  
  <pre><code>var startTime = NSDate.timeIntervalSinceReferenceDate()
       var currentTime: NSNumber = 0
  
      for var i = 0; i &lt; 10000; i++ {
          if i == 99 {
              currentTime = NSDate.timeIntervalSinceReferenceDate()
              println(i)
          }
  
      }
      var elapsedTime = currentTime - startTime
      println(Double(elapsedTime))
  }
  </code></pre>
  
  <p>and i get this error : </p>
  
  <pre><code>Cannot invoke '-' with an argument list of type '(@!Value NSNumber, @!Value NSTimerinerval)
  </code></pre>
  
  <p>i understand it's something with the type i jutt cant figure it out, any suggestions?</p>