System date countdown c#

Hi. I'm trying to make a countdown that continue even if the app is closed. In editor works fine. Everything. But the problem is when I compile the apk for my Android. On android it don't work and I really can't figure why. Can anyone help me ? Here is my script:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 using System;
 
 public class CountDown : MonoBehaviour {
 
     DateTime currentDate;
     DateTime oldDate;
     public float FinishTime = 1.0f;
     public Text FinishTimeText;
     public Text GoneText;
     public float GoneTime = 0.0f;
     public bool startedProcess = false;
     void Start () {
         currentDate = System.DateTime.Now;
         long temp = Convert.ToInt64(PlayerPrefs.GetString("sysString"));
         DateTime oldDate = DateTime.FromBinary(temp);
         TimeSpan difference = currentDate.Subtract(oldDate);
         print("Difference: " + difference);
 
         FinishTime -= (difference.Seconds) + (difference.Minutes * 60) + (difference.Hours * 60 * 60) + (difference.Days * 60 * 60 * 24);
         GoneTime = (difference.Seconds) + (difference.Minutes * 60) + (difference.Hours * 60 * 60) + (difference.Days * 60 * 60 * 24);
     }
     
     // Update is called once per frame
     void Update () {
         GoneText.text = GoneTime.ToString ();
         var timeSpan = System.TimeSpan.FromSeconds(FinishTime);
         if(FinishTime > 3600){
             FinishTimeText.text = timeSpan.Hours.ToString() + "H" + " " + timeSpan.Minutes.ToString() + "M" + " " + timeSpan.Seconds.ToString() + "S";
         }
         else if(FinishTime > 60 && FinishTime < 3600){
             FinishTimeText.text = timeSpan.Minutes.ToString() + "M" + " " + timeSpan.Seconds.ToString() + "S";
         }
         else if(FinishTime > 1 && FinishTime < 60){
             FinishTimeText.text = timeSpan.Seconds.ToString() + "S";
         }
         else if(FinishTime < 1){
             FinishTimeText.text = "Completed !";
         }
 
         if(startedProcess){
             FinishTime = Mathf.Max (0, FinishTime-Time.deltaTime);
 
             if(FinishTime <= 0)
             {
                 print("Finished");
                 startedProcess = false;
             }
         }
     }
 
     public void StartCountDown(){
         print("Started");
         startedProcess = true;
     }
     void OnApplicationQuit()
     {
         //Savee the current system time as a string in the player prefs class
         PlayerPrefs.SetString("sysString", System.DateTime.Now.ToBinary().ToString());
 
         print("Saving this date to prefs: " + System.DateTime.Now);
     }
 }