Timer.cs

using System;
using UnityEngine;
using TMPro;

public class Timer : MonoBehaviour
{
    [SerializeField] private TextMeshProUGUI timerText;
    [SerializeField] private float countdownValue = 60f;

    DateTime targetTime;
    double currentTime;
    bool active = false;

    private void Start()
    {
        // If the timer was started, load the value
        if(PlayerPrefs.HasKey("targetTime"))
        {
            targetTime = DateTime.Parse(PlayerPrefs.GetString("targetTime"));
            active = true;
        }
    }

    public void StartTimer()
    {
        // Calculate when the timer will elapse by adding the countdownValue to the current time, to get the targetTime
        targetTime = DateTime.Now.Add(TimeSpan.FromSeconds(countdownValue));
        PlayerPrefs.SetString("targetTime", targetTime.ToString());
        active = true;
    }

    public void StopTimer()
    {
        currentTime = 0f;
        active = false;
    }

    // Update is called once per frame
    void Update()
    {
        // This is just a simple way to start the timer initially
        if (Input.GetKeyDown(KeyCode.Y))
        {
            StartTimer();
        }

        // If the timer is not active, don't update it anymore
        if (!active)
            return;

        currentTime = (targetTime - DateTime.Now).TotalSeconds;

        if (currentTime <= 0)
        {
            StopTimer();
        }

        TimeSpan t = TimeSpan.FromSeconds(currentTime);
        timerText.text = t.ToString(@"mm\:ss");
    }
}