How to call time function once time?

Hi everybody. I making a countdown clock same as candycrush. It means that if you play over turn you will have to wait a few minutes to have next turn. My problem is when i can't not find the way to set a CDTime , it's auto update a variable from system time, so realtime will never bigger than CDTime. Please help .Me.hope to see your solution. Many thanks.

P/s: i find that when i used onclick button CDTime didn't update system time variable, but i don't want to use button.

void FixedUpdate() { turnUsed = +gameOverScript.turnPlayed; Debug.Log(turnUsed);

     if (turnUsed >= turnHave)
     {
         
     }
     countDown();

 }
 public void startCountDown()
 {

     System.DateTime CDDate = new System.DateTime();
     CDDate = System.DateTime.Now;
     CDTime = (CDDate.Hour + hours) * 60 * 60 + (CDDate.Minute + minutes) * 60 + (CDDate.Second + second);

 }
 public void countDown()
 {

     if (dayTime >= CDTime)
     {
         Debug.Log("endCountDown");
         startCount = false;
     }
     else Debug.Log("noCount");
 }