How to check timer user in game

Code (CSharp):

1.     void Update() {
2.  
3.         if(timeCheck) {
4.             timeInGame = Time.realtimeSinceStartup;
5.         }
6.  
7.     }
8.  
9.     void OnApplicationPause(bool pauseStatus) {
10.  
11.         if(pauseStatus && pauseDetect) {
12.  
13.             timeCheck = false;
14.  
15.         } else if(!pauseStatus && pauseDetect) {
16.  
17.             timeCheck = true;
18.  
19.         } else {
20.  
21.             pauseDetect = true;
22.            
23.         }
24.  
25.     }

I use function Time.realtimeSinceStartup for timer user in my game. But Time.realtimeSinceStartup It's active always. It can run when app go to background. I want to reset to zero when user run app from background again.