Cannot apply indexing with [] to an expression of type `UnityEngine.Transform'

I have 2 errors 1.Assets/gametime.cs(29,57): error CS1061: Type UnityEngine.Transform' does not contain a definition for Length' and no extension method Length' of type UnityEngine.Transform' could be found (are you missing a using directive or an assembly reference?) 2.Assets/gametime.cs(30,25): error CS0021: Cannot apply indexing with [] to an expression of type `UnityEngine.Transform'

using UnityEngine;

using System.Collections;

public class gametime : MonoBehaviour {

 public Transform DirectionalLight;
 public float dayCycleInMinutes = 1;

 private const float SECOND = 1;
 private const float MINUTE = 60 * SECOND;
 private const float HOUR = 60 * MINUTE;
 private const float DAY = 24 * HOUR;

 private const float DEGREES_PER_SECOND = 360 / DAY;

     private float _degreeRotation;

 private float _timeOfDay;


 // Use this for initialization
 void Start () {
     _timeOfDay = 0;
     _degreeRotation = DEGREES_PER_SECOND * DAY / (dayCycleInMinutes * MINUTE);
 
 }
 
 // Update is called once per frame
 void Update () {
     for(int cnt = 0; cnt < DirectionalLight.Length; cnt ++)  
         DirectionalLight [cnt].Rotate(new Vector3(_degreeRotation, 0, 0) * Time.deltaTime);

     _timeOfDay += Time.deltaTime;
     Debug.Log (_timeOfDay);
 
 }

}