Rotate a GameObject slowly until reach some point


I have a gameObject in my scene that is rotating using this code transform.Rotate(Vector3.up * Time.deltaTime * 15f);

then I did another method that rotate the Object to be in front of my target using this code Object.transform.LookAt(target);

but what i wanted to do is make this move slowly so the user can see that rotation


public class Rotation : MonoBehaviour
private Quaternion _startRotation;
private Quaternion _targetRotation;
private float _rotationTime;

	public Transform TargetObject; // you must to set the target object in the editor or code
	public float TimeToRotation = 3f;

	void Start()
		_startRotation = transform.rotation;
		_targetRotation = transform.rotation;
		transform.rotation = _startRotation;
		_rotationTime = 0f;// reset it to zero if you want to start the new rotation
	void Update()
		_rotationTime += Time.deltaTime;
		transform.rotation = Quaternion.Slerp(
			_startRotation, _targetRotation, _rotationTime / TimeToRotation);