I need a 2 hour realtime timer for my game

 private int Gems;
     public GameObject Main;
     private int Coins;
     public GameObject NoEnough;
     private string B;
     public GameObject BuyManagerObject;
     public Text TimeText;
     public float mstoWait;
     public GameObject Bo;
     private string Booster;
     private ulong lastbooster;
     private void Start()
     {
         //lastbooster = ulong.Parse(PlayerPrefs.GetString("LastBooster"));
         Booster = PlayerPrefs.GetString("BoosterAct");
         lastbooster = ulong.Parse (PlayerPrefs.GetString("LastBooster"));
         if (!IsChestReady())
         {
             Main.SetActive(true);
             PlayerPrefs.SetString("BoosterAct", "true");
            
         }
 
     }
    private bool IsChestReady()
     {
         ulong diff = ((ulong)DateTime.Now.Ticks - lastbooster);
         ulong m = diff / TimeSpan.TicksPerMillisecond;
         float SecondsLeft = (float)(mstoWait - m) / 1000.0f;
      if (SecondsLeft < 0)
             return true;
 
         return false;
     }
     public void Gems450(){
         Gems = PlayerPrefs.GetInt("NoofDiamond");
         if(Gems <= 450)
         {
             Gems -= 450;
             PlayerPrefs.SetInt("NoofDiamond", Gems);
             lastbooster = (ulong)DateTime.Now.Ticks;
             PlayerPrefs.SetString("LastBooster",lastbooster.ToString());
             PlayerPrefs.SetString("BoosterAct", "true");
             PlayerPrefs.SetInt("BoosterTimer", 7200);
             PlayerPrefs.SetInt("Type",1);
             Bo.SetActive(false);
             Main.SetActive(true);
             Set();
         }
         else
         {
             NoEnough.SetActive(true);
         }
 }
     public void OK()
     {
         PlayerPrefs.SetInt("N",0);
     }
     private void Update()
     {
         Booster = PlayerPrefs.GetString("BoosterAct");
         if (Booster == "true")
         {   
             
             ulong diff = ((ulong)DateTime.Now.Ticks - lastbooster);
             ulong m = diff / TimeSpan.TicksPerMillisecond;
             float SecondsLeft = (float)(mstoWait - m) / 1000.0f;
             string r = "";
             //hour
             r += ((int)SecondsLeft / 3600).ToString() + "h ";
             SecondsLeft -= ((int)SecondsLeft / 3600) * 3600;
             //minutes
             r += ((int)SecondsLeft / 60).ToString("00") + "m ";
             //Seconds
             r += (SecondsLeft % 60).ToString("00") + "s ";
             TimeText.text = r;
             PlayerPrefs.SetString("BoosterAct","true");
             Booster = PlayerPrefs.GetString("BoosterAct");
             Bo.SetActive(false);
             Main.SetActive(true);
             if (IsChestReady())
             {
                 PlayerPrefs.SetString("BoosterAct", "false");
             }
 
         }
         
         else
         {
             Bo.SetActive(true);
             Main.SetActive(false);
            
         }
     }
     public void Set()
     {
         mstoWait = PlayerPrefs.GetInt("BoosterTimer");
         Debug.Log(mstoWait);
         Main.SetActive(true);
     }

This was the code I used but It Didnt woark properly. The Trouble Is caused while the scene Starts. The variable 'SecondsLeft ' Gets back to zero. Can Some One Help Me please. Thank you