Timer.Swift

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var timerLabel: UILabel!

    var countdownTimer: Timer!
    var totalTime = 60 // In Sec


    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    func startTimer() {
        countdownTimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(updateTime), userInfo: nil, repeats: true)
    }

    func updateTime() {
        timerLabel.text = "\(timeFormatted(totalTime))"

        if totalTime != 0 {
            totalTime -= 1
        } else {
            endTimer()
        }
    }

    func endTimer() {
        countdownTimer.invalidate()
    }

    func timeFormatted(_ totalSeconds: Int) -> String {
        let seconds: Int = totalSeconds % 60
        let minutes: Int = (totalSeconds / 60) % 60
        //     let hours: Int = totalSeconds / 3600
        return String(format: "%02d:%02d", minutes, seconds)
    }


    @IBAction func startTimerPressed(_ sender: UIButton) {
        startTimer()
    }

}