Start timer on mouse0 click

I want to start a timer when i click mouse0 (lmb), so that on only one click the timer starts counting.

What i have now only adds a few splits of a second to the timer for every click. It does not “count up”.

if (Input.GetKeyDown (KeyCode.Mouse0))
		{
			isWithdrawing = true;
			WithdrawTimer += Time.deltaTime;
		}

The variables used are public so i can confirm it.

Any thoughts on how to make it count up fluently when i click the mouse0 key?

Thank you

I added another bool and put the timerlogic in a function of its own.
On mouse click the bool is turned true and wil InvokeRepeat the TimerLogic. When you want the timer to stop you have to put bool"X" to false.

 using UnityEngine;
    using System.Collections;

public class Timer : MonoBehaviour {

	bool isWithdrawing; 
	bool X = false;
	float WithdrawTimer;
	public GUIText timerText;

	// Use this for initialization
	void Start () {
	
	}

	void TimerLogic(){
		isWithdrawing = true;
		WithdrawTimer += Time.deltaTime;
	}
	
	// Update is called once per frame
	void Update () {
		timerText.text = WithdrawTimer.ToString ();
	
		if (Input.GetKeyDown (KeyCode.Mouse0))
		{
			X= true;
		}

		if (X == true) {
			InvokeRepeating("TimerLogic",0.1f,60.0f);
		}
else 
{
CancelInvoke("TimerLogic")
}
	}
}