Keep Getting NullReferenceException: Object reference not set to an instance of an object

this is super annoying and I cant seem to understand why im getting it, someone explain it please?

Heres the line that gives me the error;

txtDate.text = DateNumber.ToString("0");

and heres the whole script;

 using UnityEngine;
 
 using System.Collections;
 
 using UnityEngine.UI;
 
 public class CalanderScript : MonoBehaviour {
 
     public Text txtDate;
     private float DateNumber;
     public int CapValue;
     public int TimeValue;
 
     void Start () {
 
         txtDate = GetComponent<Text>();
         DateNumber = 1;
     
     }
     
     // Update is called once per frame
     void Update()
     {
         
         DateNumber += Time.deltaTime / TimeValue;
         txtDate.text = DateNumber.ToString("0");
 
         if (DateNumber >= CapValue)
         {
             DateNumber = 1;
         }
 
     }
 
 
 }