timer-outlet.swift

@IBAction func stopButton(_ sender: Any) {        
        timer.invalidate()          
}
       
@IBAction func resetButton(_ sender: Any) {       
        startNumber = 240
        numberLabel.text = String(startNumber)
        timer.invalidate()        
}
    
@IBAction func plusTimeButton(_ sender: Any) {        
        startNumber += 10
        numberLabel.text = String(startNumber)       
}
    
@IBAction func subTimeButton(_ sender: Any) {        
        if startNumber >= 10 {
            startNumber -= 10
            numberLabel.text = String(startNumber)
        }
}