Clock doesn't stop when reaches 0.0. Can you help me with this.

So I have the problem that I never reach the else statement in update. And also i want the clock to stop at 0.0. The startTime is in seconds. If you make it 90.0f you will have 1.30minute. The problem is i need to stop this clock when reaches the 0:0.0. Thank you in advance

public class TImer : MonoBehaviour
{

    public Text TimerText;
    private float startTime = 3.0f;
    private bool start = false;
    private float _time;
    private float _minutes, _seconds;

	// Use this for initialization
	void Start ()
	{
	    start = false;
	    startTime = 3.0f;
	}
	
	// Update is called once per frame
	void Update ()
	{
	   // if (start)
	  //  return;
	    if (startTime > 0.0f)
	    {
            _time = startTime - Time.time; // ammount of time since the time has started
            _minutes = (int)_time / 60;
            _seconds = _time % 60;
            TimerText.text = _minutes + ":" + _seconds.ToString("f1");
        }
	    else
	        Debug.Log("we are here"); 
    }

    private void CheckGameOver()
    {
        Debug.Log("gameover");
    }

    public void StartTime()
    {
        TimerText.color = Color.black;
        start = true;
    }

Fixed it.
so in start method I made that time is startTime

oid Start ()
	{
	    start = false;
	    _time = startTime;
	}

And in Update method I said if _time is bigger then 0.0f then run clock and stop if it’s less.

void Update ()
	{
	   // if (start)
	  //  return;
	    if (_time > 0.0f)
	    {
            _time = startTime - Time.time; // ammount of time since the time has started
            _minutes = (int)_time / 60;
            _seconds = _time % 60;
            TimerText.text = _minutes + ":" + _seconds.ToString("f1");
        }
	    else
	    {
	        CheckGameOver();
	    }
    }