How do i start a new objective and timer C#

Hi everyone, i'm creating a new game for school and i have some issues starting up. This is my first game ever in Unity3D and i have hit a speedbump on my first attemt at scripting. Please take in mind that i use C# because i need to for school and i cannot use Javascript.

My idea for the game is that it is going to be something like a pizza delivery game. When the game starts u get an objective to drive to in the most efficient way u can. My problem is as followed.

I need to tell the game it has a new objective and i need to calculate the time taken from start to delivery. Then i need to let a formula have a go at my delivery time giving me a certain amount of points for the delivery. the formula is: (1800/"deliveryTime")+30 When the delivery is complete the player needs to drive back to a certain point in the map to start a new objective/delivery.

So far i got this:

using UnityEngine;
using System.Collections;

public class timer : MonoBehaviour {

    public float startDelTime = 0.0f;
    public float endDelTime = 0.0f;
    public bool startNewDelivery
    public bool deliveryFinished;

        void Start () {

            startNewDelivery {

            }

    }

    void Update () {

        endDelTime += Time.deltaTime;

        if(deliveryFinished){
            Debug.Log("Game Over");
        }

    }
}

I don't know if this makes sense but i'm like in over my head with this and i could really use some help. BTW i already got a drivable car.

You don't sound very clear to me, but my guess is that change this script into this

using UnityEngine;
using System.Collection;

public class timer : MonoBehaviour
{
      private startTime;
      private int restSeconds;
      private int roundedRestSeconds;
      private int displaySeconds;
      private int displayMinutes;
      //how much time the player has.
      static int countDownSeconds = 200;

      void Awake ()
      {
            startTime = Time.time;
      } 
      void OnGUI ()
      {
            private guiTime = Time.time - startTime;
            restSeconds = countDownSeconds - (guiTime);

            roundedRestSeconds = Mathf.CeilToInt(restSeconds);
            displaySeconds = roundedRestSeconds % 60;
            displayMinutes = roundedRestSeconds / 60;

            text = String.Format ("{0:00}:{1:00}", displayMinutes, displaySeconds);
            if(restSeconds==0)
            {
                  Time.timeScale = 0;
                  print("Game Over");
            }
      }
}

(Note that there may be some errors becuase this script was originally JavaScript) And then make a trigger at the destination with a script saying -

using UnityEngine;
using System.Collection;

public class trigger : MonoBehavior 
{
      public int addTime = 30;

      void OnTriggerEnter (Collider collider)
      {
            timer.countDownSeconds += addTime;
            Destroy(gameObject);
      }
}

Hopes this helps!