Stop Watch 질문

Stop Watch 강의도중 시간, 분, 초를 출력해보고 싶어서 코드를 짯습니다.

근데 start 버튼을 누르면 바로 hour부분이 09가 됩니다. 

코드입니다.

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var labelHour: UILabel!

    @IBOutlet weak var labelMinute: UILabel!

    @IBOutlet weak var labelSecond: UILabel!

    @IBOutlet weak var startStopButton: UIButton!

    @IBOutlet weak var resetButton: UIButton!

    var isStart = false

    var timer = Timer()

    var startTime = 0.0

    var elapsed = 0.0

    @IBAction func startStop(_ sender: Any) {

        if isStart{

            startStopButton.setTitle("Start", for: .normal)

            timer.invalidate()

        }else{

            startStopButton.setTitle("Stop", for: .normal)

            startTime = Date().timeIntervalSince1970 - elapsed

            timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(updateTime), userInfo: nil, repeats: true)

        }

        isStart.toggle()

    }

    @IBAction func reset(_ sender: Any) {

        labelHour.text = "00"

        labelMinute.text = "00"

        labelSecond.text = "00"

        elapsed = 0.0

        startTime = Date().timeIntervalSince1970 - elapsed

        startStopButton.setTitle("Start", for: .normal)

        timer.invalidate()

        isStart.toggle()

    }

    override func viewDidLoad() {

        super.viewDidLoad()

        labelHour.text = "00"

        labelMinute.text = "00"

        labelSecond.text = "00"

        startStopButton.setTitle("Start", for: .normal)

        resetButton.setTitle("Reset", for: .normal)

    }

    @objc func updateTime(){

        elapsed = Date().timeIntervalSince1970 - startTime

        let df = DateFormatter()

        let date = Date(timeIntervalSince1970: elapsed)

        df.dateFormat = "HH"

        labelHour.text = df.string(from: date)

        df.dateFormat = "mm"

        labelMinute.text = df.string(from: date)

        df.dateFormat = "ss"

        labelSecond.text = df.string(from: date)

    }

}

아래 사진은 5분 28초 경과 사진입니다.