Are you using C#? As long as you are using System you should have access to the DateTime libraries.

Here's a simple script I wrote in response to your question. This should get you started. This example will show the current system time in 12 hour (am/pm) format. This example is in JavaScript (UnityScript). Just attach this script to either the main camera, or some empty game object in scene. This will show the time both on the GUI, and will print to the console.

#pragma strict

    private var time : System.DateTime;
    private var h : float;
    private var m : float;
    private var s : float;
    private var format : String = "hh:mm:ss";
    private var ampm : String;

    function Update () 
    {
       time = System.DateTime.Now;
       h = time.Hour;
       m = time.Minute;
       s = time.Second;
       if(h > 0 && h < 12)
         ampm = " AM";
       else if (h > 11 && h < 24)
         ampm = " PM";
       else if(h == 0)
        ampm = " AM";
       print (h);
    }

    function OnGUI()
    {
       GUI.Box(Rect(Screen.width /2, Screen.height /2, Screen.width /10, Screen.height /10), time.ToString(format)+ampm);
    }

This was a piece of javescript that I was using to grab the date once a long time ago I modified it to do the time.

This Link will give you all the different ways you can use System.DateTime in a string

http://www.geekzilla.co.uk/View00FF7904-B510-468C-A2C8-F859AA20581F.htm

//sets something to record the date and time
var date : System.DateTime;

//sets something allowing you to populate it with the date and time
var text : String;


function Update()
{
    //tells the var to look for the precise moment you are at
    var date = System.DateTime.Now;
    //converts the above var to a readable string
    text = date.ToString("HH:mm:ss");
    //prints it to inspector
    print (text);

}