here's a bit of code that might help:

public float maxlifetime = 10.0f; //you can edit this in the inspector to change how long your life should be
private float timespent = 0.0f; //this is basically how long you have been alive for
private bool isdead = false; //boolean that shows if you're alive or not.

public void Update(){
    timespent += time.deltatime; //time.deltatime is the time between each frame, update is called once every frame.
    if(timespent > maxlifetime){//when timespent gets higher than maxlifetime you die.
        isdead = true;
    }
}

you can do a print on the "timespent" in the gui to see how long you've been alive for, or how long you have left, though im not sure on how you can make it show up as hours:minutes:seconds. This piece of code isnt exactly what you'd call optimal, but it might work :)

private float seconds = 0.0f;
private float minutes = 0.0f;
private float hours = 0.0f;

public void Update(){
    seconds += time.deltatime;
    if(seconds > 60){
        minutes += 1;
        seconds = 0;
    }
    if(minutes > 60{
        hours += 1;
        minutes = 0;
    }
    print("Hours: " + hours + " " + "Minutes: " + minutes + " " + "Seconds" + seconds);

}

again, there is probably some smarter way, but this one should work.

though, if you want to use unity you should learn some scripting, as you cant rely on the community to make all your scripts for you.