【swift】 timer date label 画面遷移をしても継続してカウントダウンしていて欲しい

swift
1import UIKit
2
3class NestViewController: UIViewController {
4
5    var count = 0
6    var timer : Timer?
7    
8    var date : Date?
9    var date2 : Date?
10   
11     var nowNumber = 0
12    
13    var ccccc : Double  = 0
14    
15    @IBOutlet weak var countLabel: UILabel!
16    
17    @IBOutlet weak var currentLabel: UILabel!
18    
19    override func viewDidLoad() {
20        super.viewDidLoad()
21
22       
23        
24        
25    }
26    
27    override func viewWillAppear(_ animated: Bool) {
28        super.viewDidDisappear(animated)
29     
30        
31
32    }
33
34
35    @IBAction func startButton(_ sender: UIButton) {
36
37        
38        if timer?.isValid != true {
39            
40        
41        if nowNumber == 20 || nowNumber == 30 {
42            if nowNumber == 20 {
43                date = Date()
44                timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(timerInterruptTwenty(_:)), userInfo: nil, repeats: true)
45                
46            } else if nowNumber == 30 {
47                timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(timerInterruptThirty(_:)), userInfo: nil, repeats: true)
48            }
49            
50        }
51       }
52        
53    }
54    
55    @IBAction func stopButton(_ sender: UIButton) {
56
57    }
58    
59    @IBAction func twentyButton(_ sender: UIButton) {
60        labelUpdate(1200)
61        nowNumber = 20
62    }
63    
64    @IBAction func thirtyButton(_ sender: UIButton) {
65        labelUpdate(1800)
66        nowNumber = 30
67    }
68    
69    @objc func timerInterruptTwenty(_ timer : Timer) {
70        count += 1
71        date2 = Date()
72        let datenosa = date2!.timeIntervalSince(date!)
73     
74       ccccc = 1200.00 - datenosa
75        
76        labelUpdate(Int(ccccc))
77     
78        if datenosa >= 1200 {
79            count = 0
80            timer.invalidate()
81        }
82    }
83    
84    @objc func timerInterruptThirty(_ timer : Timer) {
85        count += 1
86        date2 = Date()
87        let datenosa = date2!.timeIntervalSince(date!)
88        
89        if datenosa >= 1800 {
90            count = 0
91            timer.invalidate()
92        }
93        
94    }
95    
96    
97    
98    
99    
100    func labelUpdate (_ aaa : Int) {
101        let s = aaa % 60
102        let m = (aaa - s) / 60
103        currentLabel.text = String(format: "%02d : %02d", m,s)
104        
105    }
106    
107 
108}
109