How to add your own time for your digital clock

So i have this timer script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class clockStim : MonoBehaviour
 {
   public Text timer;
   public float hour, minutes;
   public float speed = 1;
   float clockSpeed;
     // Start is called before the first frame update
     void Start()
     {
         timer = GetComponent<Text>() as Text;
     }
 
     // Update is called once per frame
     void Update()
     {
       clockSpeed += Time.deltaTime * speed;
         hour = Mathf.Floor((int)(clockSpeed % 216000) / 3600);
         minutes = Mathf.Floor((int)(clockSpeed % 3600) /60);
         timer.text = hour.ToString("00") + ":" + minutes.ToString("00");
     }
 }
 

and i want to add my own time like hours, minutes in the inspector (for different game scene) so far every time i press play, the timer reset and everything start counting from zero. please help, and thank you