how to pop interstital once a day?

I am using this for a prize each day:

 #pragma strict
 
 private var currentDate : int;
 
 private var DailyPrizeAmount : int;
 
 static var ClaimThePrize : boolean = true;
 
     
 function OnLevelWasLoaded(Level : int){
 
         if(Level == 5){
 
             if(PlayerPrefs.HasKey("IsFirstLaunch")){
 
                 //Store the current time when it starts
                 currentDate = System.DateTime.Now.get_Day();
                 print("currentDate: " + currentDate);
 
                 //Grab the old time from the player prefs.
                 var oldDate : int = PlayerPrefs.GetInt("Day");
                 print("oldDate: " + oldDate);
 
                 if(currentDate != oldDate){
                 
                     ClaimThePrize = true;
                     ShowPrize();
                     print("The Prize is Right");
                 }
             }
         
             //First launch.
             if(ClaimThePrize == true){
         
                 ShowPrize();
             }
         }
     }
     
 function OnMouseDown(){
 
         ThePrizeIsRight();
     }
     
 function ThePrizeIsRight(){
 
         ClaimThePrize = false;
         HidePrize();    
         Save();
     }                            
     
     
 function OnApplicationPause(){
 
         Save();    
     }    
  
 function OnApplicationQuit(){
 
         Save();
     }
     
 function Save(){
 
         if(ClaimThePrize == false){
 
             //Save the current system time.
             PlayerPrefs.SetInt("Day", System.DateTime.Now.get_Day());
          
             print("Saving this date to prefs: " + System.DateTime.Now.get_Day());
             //Debug.Log(Application.persistentDataPath);
         }
     }
     
 function ShowPrize(){
 
         DailyPrizeAmount = Random.Range(100000,150000);
  
     }
     
 function HidePrize(){
 
         Earnings.TotalEarnings += DailyPrizeAmount;
     }                                                        
     
     
 /*    
 var theTime : String = System.DateTime.Now.ToString("hh:mm:ss"); 
 
 var theDate : String = System.DateTime.Now.ToString("MM/dd/yyyy"); 
 
 var theMonth : int = System.DateTime.Now.get_Month(); 
 
 var theDay : int = System.DateTime.Now.get_Day();
 */