Set a Timer countdown from a TimeSpan

Hi! I've been dealing with this for two days and I don't know what else to try so, hoping someone can help. I have set a timer which shows the remaining hours until the player can access the Level again by comparing the time difference between GameOverTime and StartTime (both saved on PlayerPrefs; which I know it's not the safest option but it has to be like this for now). My problem is that I want that timer to keep counting down while it's being displayed on screen. And I don't know how to do that. Basically, if the remaining time is 5:15:08 I want it to keep going down from there to 5:15:07, 5:15:06, etc. I've tried using several things I've found online but none has worked so far :/

The latest thing I came up with was to provide the player with a button to update the time (kind of a "refresh page" thingy) but the way I have it, it doesn't work either...

Here is my code:

private bool unlocked; public string level;

 public GameObject levelLocked;
 public GameObject holder;
 public GameObject holder2;
 public GameObject holder3;


 public GameObject timeLeftObject;
 public DateTime startTime;
 public DateTime GameOverTime;
 public TimeSpan timeDifference;
 public Text timer;
 private String timeRemaining;
 private TimeSpan timeDifference2;
 public TimeSpan timeToPass = new TimeSpan (8,0,0);
 public TimeSpan timeToRestart1;


 // Use this for initialization
 void Start () {

     PlayerPrefs.SetInt ("Level1", 1);
     startTime = DateTime.Now;

     GameOverTime = Convert.ToDateTime(PlayerPrefs.GetString ("LastTimeGameOver"));
     Debug.Log(GameOverTime);

     timeDifference = startTime - GameOverTime;
     timeToRestart1 = timeDifference.Subtract (timeToPass);


     Debug.Log (timeDifference);

     timer = GetComponent<Text> ();
     timeRemaining = string.Format ("{0:D2}:{1:D2}:{2:D2}", Math.Abs(timeToRestart1.Hours), Math.Abs(timeToRestart1.Minutes), Math.Abs(timeToRestart1.Seconds));

     Debug.Log ("Ha cambiado");


     if (PlayerPrefs.GetInt ("Level21") == 1) {

         holder.SetActive (false);
         holder2.SetActive (true);
     }

     if (PlayerPrefs.GetInt ("Level41") == 1) {

         holder.SetActive (false);
         holder2.SetActive (false);
         holder3.SetActive (true);

     } else {
         
         holder.SetActive (true);
         holder2.SetActive (false);
         holder3.SetActive (false);

 } 

}

 // Update is called once per frame
 void Update () {
     


     if (timeDifference.Hours < 8) {

         timeLeftObject.SetActive (true);
         timer.text = timeRemaining;

         Debug.Log ("Menor que 8 horas");
         
     } if(timeDifference.Hours >= 8){

         timeLeftObject.SetActive (false);
         Debug.Log ("Más o igual a 8 horas");
     }
 
 }
         

 public void SelectLevel (string level)
 {
         
     if (PlayerPrefs.GetInt(level) == 1){

         unlocked = true;
         SceneManager.LoadScene (level);

     } else {

         unlocked = false;
         levelLocked.SetActive (true);

         if (Input.GetKeyDown(KeyCode.Space)) {

             levelLocked.SetActive (false);
             unlocked = false;
         }
     }
 }

 public void UpdateTime(){

     if (Input.GetKeyDown (KeyCode.Space) || Input.GetMouseButtonDown(0)) {

         timer.text = timeRemaining;
     }
 }

 }
     

I'm a noob in programming so maybe the answer is really simple... Any help is very much appreciated! Thanks!