Swift - cannot call '-' with argument list like (NSNumber, NSTimeinterval)

I am trying to get the current time from a specific point from the starting point of the application.

var startTime = NSDate.timeIntervalSinceReferenceDate()
     var currentTime: NSNumber = 0

    for var i = 0; i < 10000; i++ {
        if i == 99 {
            currentTime = NSDate.timeIntervalSinceReferenceDate()
            println(i)
        }

    }
    var elapsedTime = currentTime - startTime
    println(Double(elapsedTime))
}

      

        
        
        
      

    

and I am getting this error:

Cannot invoke '-' with an argument list of type '(@!Value NSNumber, @!Value NSTimerinerval)

      

        
        
        
      

    

I understand this is something with the type I jutt can not, is that all, any suggestions?