works on swift5.0
var timer = Timer()
var secondsPassed = 0
var totalTime = 300
timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(self.update), userInfo: nil, repeats: true)
@objc func update() { if secondsPassed < totalTime{ secondsPassed += 1 }else{ timer.invalidate() print("Time's up") }

@IBAction func start(sender: UIButton) { self.timer = NSTimer.scheduledTimerWithTimeInterval(1.0, target: self, selector: #selector(ViewController.update(_:)), userInfo: nil, repeats: true) NSRunLoop.currentRunLoop().addTimer(self.timer, forMode: NSRunLoopCommonModes) startTime = NSDate() // new instance variable that you would need to add.
}
func update() { let elapsedTime = NSDate().timeIntervalSinceDate(startTime) let currTime = totalTime - elapsedTime //total time is an instance variable that is the total amount of time in seconds that you want countDown.text = String(currTime) if currTime < 0 { timer.invalidate() //do other stuff that you need to do when time runs out. }
}

var secondsRemaining = 60
Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(updateCounter), userInfo: nil, repeats: true)
}
@objc func updateCounter(){ if secondsRemaining > 0 { print("\(secondsRemaining) seconds.") secondsRemaining -= 1 } }

var timer = NSTimer()
timer(timeInterval: 0.01, target: self, selector: update, userInfo: nil, repeats: false)