about Time.time

if I press a button,the time begins to run rather than the Script run.how to get it ? i don't know whether express my qusetion correctly or not.my English is so bad.

All right! actually,i want to make a Timer ,when press a button,the time begins to run

I've used a modified version of Sebas' answer, adding a button press to start the timer.

When you press 't', the startTime is recorded. Until that time, OnGUI() is returning early due to the ifcheck (is t still null).

Let me know if there's anything you don't understand.

private var startTime : float = 0.0;
var textTime : string;

function Update()
{
   if ( Input.GetKeyDown("t") )
   {
      startTime = Time.time;
   }
}

function OnGUI ()
{
   if ( startTime == 0 )
      return;

   var guiTime = Time.time - startTime;

   var minutes : int = guiTime / 60;
   var seconds : int = guiTime % 60;
   var fraction : int = (guiTime * 100) % 100;

   textTime = String.Format ("{0:00}:{1:00}:{2:000}", minutes, seconds, fraction); 
   GUI.Label (Rect (400, 25, 100, 30), textTime);
}