add real time

You can get the time with "Time.time".

http://docs.unity3d.com/Documentation/ScriptReference/Time.html

Well, Unity provides Time.realtimeSinceStartup if you want to have real seconds. Usually you would use Time.time. However it is scaled with Time.timeScale, that comes in handy if you want to do a slowmotion effect or you want to pause the game. realtimeSinceStartup will never stop no matter what you do in game.

Regardless of the "time base" you want to use, you have to store the start time in a variable, so you can calculate the difference from the start point.

 // UnityScript
 var startTime = 0.0f;
 var timeTaken = 0.0f;
 
 // call this function when you want to start your game
 function StartGame()
 {
     startTime = Time.time;
 }
 
 // This function will return the time in seconds that have passed since the start time
 function GetCurrentGameTime() : float
 {
     return Time.time - startTime;
 }
 
 // call this function when the game is over. it will return the points depending on the timeTaken.
 function StopGame() : int
 {
     timeTaken = GetCurrentGameTime();
     if (timeTaken > 5*60)
         return 50;
     if (timeTaken > 3*60)
         return 100;
     if (timeTaken > 1*60)
         return 150;
     return 1000;
 }

In most games it's better to calculate the points on a mathematical formula. This way the user get different points for every fraction of a second he saved.

 function StopGame() : int
 {
     timeTaken = GetCurrentGameTime();
     float t = Mathf.Clamp01(timeTaken / (5*60.0f));
     t = 1.0f - t;
     return 50.0f + t*500.0f;
 }

This will always return 50 points plus a bonus depending on the time taken if you're under 5 min. The closer you come to 0 the higher the bonus will get. If the player completes the game in 0 seconds (just in theory) he will get a bonus of 500 points.

Here's are some samples:

 timeTaken  bonus  total
 300sec     0      50      // 5 min  0 sec
 240sec     100    150     // 4 min  0 sec
 239sec     101    151     // 3 min 59 sec
 235sec     108    158     // 3 min 55 sec
 70sec      383    433     // 1 min 10 sec