The real time in seconds since the game started (Read Only).

In almost all cases you can and should use Time.time instead.

realtimeSinceStartup returns the time since startup, not affected by Time.timeScale. realtimeSinceStartup also keeps increasing while the player is paused (in the background). Using realtimeSinceStartup is useful when you want to pause the game by setting Time.timeScale to zero, but still want to be able to measure time somehow.

Note that realtimeSinceStartup returns time as reported by system timer. Depending on the platform and the hardware, it may report the same time even in several consecutive frames. If you're dividing something by time difference, take this into account (time difference may become zero!).

	// Prints the real time since start up.
	print(Time.realtimeSinceStartup);

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Example() {
        print(Time.realtimeSinceStartup);
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Example() as void:
		print(Time.realtimeSinceStartup)

Another example:

	// A FPS counter.
	// It calculates frames/second over each updateInterval,
	// so the display does not keep changing wildly.
	 
	var updateInterval = 0.5;
	private var lastInterval : double; // Last interval end time
	private var frames = 0; // Frames over current interval
	private var fps : float; // Current FPS
	function Start() {
		lastInterval = Time.realtimeSinceStartup;
		frames = 0;
	}
	function OnGUI () {
		// Display label with two fractional digits
		GUILayout.Label("" + fps.ToString("f2"));
	} 
	function Update() {
		++frames;
		var timeNow = Time.realtimeSinceStartup;
		if( timeNow > lastInterval + updateInterval )
		{
			fps = frames / (timeNow - lastInterval);
			frames = 0;
			lastInterval = timeNow;
		}
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    public float updateInterval = 0.5F;
    private double lastInterval;
    private int frames = 0;
    private float fps;
    void Start() {
        lastInterval = Time.realtimeSinceStartup;
        frames = 0;
    }
    void OnGUI() {
        GUILayout.Label("" + fps.ToString("f2"));
    }
    void Update() {
        ++frames;
        float timeNow = Time.realtimeSinceStartup;
        if (timeNow > lastInterval + updateInterval) {
            fps = frames / (timeNow - lastInterval);
            frames = 0;
            lastInterval = timeNow;
        }
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	public updateInterval as float = 0.5F

	private lastInterval as double

	private frames as int = 0

	private fps as float

	def Start() as void:
		lastInterval = Time.realtimeSinceStartup
		frames = 0

	def OnGUI() as void:
		GUILayout.Label(('' + fps.ToString('f2')))

	def Update() as void:
		++frames
		timeNow as float = Time.realtimeSinceStartup
		if timeNow > (lastInterval + updateInterval):
			fps = (frames / (timeNow - lastInterval))
			frames = 0
			lastInterval = timeNow