Swift 画面遷移しても継続してカウントダウンして欲しい。

UserDefaultを使ってみるも良く分からず。。。

import UIKit

class NextViewController: UIViewController {

    var timer : Timer?
    var date1 : Date?
    var date2 : Date?

    var count = 15.0

    @IBOutlet weak var countDownLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    @IBAction func startButton(_ sender: UIButton) {
        date1 = Date()

        if timer?.isValid != true {
            timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(fifteenCount(_:)), userInfo: nil, repeats: true)
        }
    }

    @objc func fifteenCount (_ timer : Timer) {

        date2 = Date()
        let dateDifference = date2!.timeIntervalSince(date1!)

        count = 15 - dateDifference

        countDownLabel.text = String(Int(count))

        if count <= 0 {
             timer.invalidate()
        }

    }
}

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