Time difference

Hi i want to saving time when player will pause the game and when he qiut the game but problem is that when i quit from the game that time is not saving and i dont know why but if i pause that game time is saving so can u help me?

 void Start()
 {
     if (difference2.TotalSeconds == 0)
     {
         NowTime = System.DateTime.Now;

         long temp = Convert.ToInt64(PlayerPrefs.GetString("wtf"));

         OldTime = DateTime.FromBinary(temp);

         difference = NowTime.Subtract(OldTime);
         now.text = "roznica wynosi: " + difference.TotalSeconds + "\n Aktualny: " + System.DateTime.Now.ToString("HH:mm:ss") + "\n Ostatni: " + OldTime.Hour + ":" + OldTime.Minute + ":" + OldTime.Second;
     }
     }
 void OnApplicationQuit()
 {
     PlayerPrefs.SetString("wtf", System.DateTime.Now.ToBinary().ToString());
 }
 void OnApplicationFocus()
 {
   if (difference.TotalSeconds == 0)
     {
         NowTime = System.DateTime.Now;

         long temp = Convert.ToInt64(PlayerPrefs.GetString("wtf2"));

         OldTime = DateTime.FromBinary(temp);

         difference2 = NowTime.Subtract(OldTime);
         now.text = "roznica wynosi: " + difference2.TotalSeconds + "\n Aktualny: " + System.DateTime.Now.ToString("HH:mm:ss") + "\n Ostatni: " + OldTime.Hour + ":" + OldTime.Minute + ":" + OldTime.Second;

     }

 }
 void OnApplicationPause()
 {
     PlayerPrefs.SetString("wtf2", System.DateTime.Now.ToBinary().ToString());
 }