タイマーのバックグラウンド再生

タイマーのアプリを作成しているのですが、バックグラウンド再生ができません。以下のコードについてどのように改善すべきか教えていただけると幸いです。
ネットの記事を参考にしながら以下のようにコードを書きました

swift
1    var time: [Int] = [0,0,0]
2    var starttime: [Int] = [0,0,0]    
3    var stoptime: [Int] = []
4    
5    let hourlist: [Int] = Array(0...23)
6    var minlist: [Int] = Array(0...59)
7    var seclist: [Int] = Array(0...59)
8
9    override func viewDidLoad() {
10        super.viewDidLoad()
11        timernum()
12    }
13
14    @objc func timernum(){
15
16             if (time[0] == 0 && time[1] == 0 && time[2] == 0) && startflg == true{
17        
18                 timer.invalidate()
19                 
20                 time = starttime
21                 startflg = false
22                 
23                 timelabel.isHidden = true
24                 timeview.isHidden = true
25                 nowlabel.isHidden = true
26                 timepicker.isHidden = false
27                 
28                 start.backgroundColor = UIColor.clear
29                 start.setTitle("Start", for: .normal)
30
31             }else if (time[0] == 0 && time[1] == 0 && time[2] == 0) {
32                
33                start.isEnabled = false
34                start.backgroundColor = UIColor.allg
35                
36             }else{
37                
38                if time[2] > 0 && startflg == true{
39                    
40                    time[2] -= 1
41                    
42                }else if startflg == true{
43                    
44                    time[2] += 59
45                    
46                    if time[1] > 0 {
47                        time[1] -= 1
48                    }else{
49                        
50                        time[1] += 59
51                        time[0] -= 1
52                        
53                        }
54                }
55                timelabel.text = String(format: "%02d",time[0]) + " : " + String(format: "%02d",time[1]) + " : " + String(format: "%02d",time[2])
56                 print("updating_now_time:" + NSDate().description)
57            }
58            }
59    //略
60
61 @IBAction func start(_ sender: Any) {
62        
63        timelabel.isHidden = false
64        nowlabel.isHidden = false
65        
66        if startflg == false{
67            startflg = true
68            
69            timelabel.text = String(format: "%02d",time[0]) + " : " + String(format: "%02d",time[1]) + " : " + String(format: "%02d",time[2])
70            
71            var nesec = Date()
72            
73            nesec = Calendar.current.date(byAdding: .second, value: time[0] * 3600 + time[1] * 60 + time[2], to: nesec)!
74            
75            print(nesec)
76            
77            let format = DateFormatter()
78            format.locale = Locale.current
79            format.dateFormat = "yyyy/MM/dd HH:mm:ss"
80            let sdate = format.string(from: nesec)
81            nowlabel.text = String(sdate)
82            
83
84            cancel.isEnabled = true
85            cancel.backgroundColor = UIColor.clear
86
87            start.setTitle("Pouse", for: .normal)
88            start.backgroundColor = UIColor.allg
89            start.titleLabel?.font = UIFont.systemFont(ofSize: 26)
90            
91            timepicker.isHidden = true
92            timeview.isHidden = false
93            
94            timeview.backgroundColor = UIColor.wb
95            self.view.addSubview(timeview)
96            
97
98            timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector:#selector(timernum) , userInfo: nil, repeats: true)
99
100        }else if startflg == true && pouseflg == false{
101            
102            pouseflg = true
103
104            stoptime = time
105            timer.invalidate()
106            
107            if stoptime[0] == 0 && stoptime[1] == 0 && stoptime[2] == 0{
108               
109            }else{
110            timelabel.text = String(format: "%02d",time[0]) + " : " + String(format: "%02d",time[1]) + " : " + String(format: "%02d",time[2])
111
112            }
113            
114            print(stoptime)
115            start.setTitle("Resume", for: .normal)
116            start.titleLabel?.font = UIFont.systemFont(ofSize: 27)
117            start.backgroundColor = UIColor.yellow
118            
119        }else if pouseflg == true{
120            
121            pouseflg = false
122            
123            start.setTitle("Pouse", for: .normal)
124            start.backgroundColor = UIColor.allg
125            start.titleLabel?.font = UIFont.systemFont(ofSize: 26)
126            
127            var nesec = Date()
128
129            nesec = Calendar.current.date(byAdding: .second, value: time[0] * 3600 + time[1] * 60 + time[2], to: nesec)!
130            
131            let format = DateFormatter()
132            format.locale = Locale.current
133            format.dateFormat = "yyyy/MM/dd HH:mm:ss"
134            let sdate = format.string(from: nesec)
135            nowlabel.text = String(sdate)
136            
137            timer = Timer.scheduledTimer(timeInterval: 1, target: self, selector:#selector(timernum) , userInfo: nil, repeats: true)            
138        }        
139    }
140

AppDelegateには以下の内容の記述をしました。

swift
1import UIKit
2
3@main
4class AppDelegate: UIResponder, UIApplicationDelegate {
5    
6    
7    var window: UIWindow?
8    var backgroundTaskID : UIBackgroundTaskIdentifier = UIBackgroundTaskIdentifier(rawValue: 0)
9
10    private func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
11
12           return true
13       }
14
15       //バックグラウンド遷移移行直前に呼ばれる
16    func applicationWillResignActive(_ application: UIApplication) {
17
18           self.backgroundTaskID = application.beginBackgroundTask(){
19               [weak self] in
20               application.endBackgroundTask((self?.backgroundTaskID)!)
21               self?.backgroundTaskID = UIBackgroundTaskIdentifier.invalid
22           }
23
24       }
25
26       //アプリがアクティブになる度に呼ばれる
27    func applicationDidBecomeActive(_ application: UIApplication) {
28
29           application.endBackgroundTask(self.backgroundTaskID)
30       }
31
32
33