How can I manage that a 3dtext show date?

Here is a simple script - just place it on your text object and link the textMesh as the reference.

 using UnityEngine;
 
 
 public class DisplayTime : MonoBehaviour
 {
     [SerializeField] TextMesh timeText;
 
     void Update()
     {
         System.DateTime current = System.DateTime.Now;
         timeText.text = string.Format("{0}/{1}/{2}", current.Day, current.Month, current.Year);
     }
 }

As a tip, I wouldn't use textmesh anymore. Everyone should be using TextMeshPro. If you are using it, then just switch out the text type.

Good luck!