LimitTimer.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using System;

public class LimitTimer : MonoBehaviour
{
    public Text timerText
    public int minute;
    public float seconds;
    private float totalTime;
    private float oldSeconds;
    private float startTime;
    

    void Start()
    {
        this.totalTime = this.minute * 60 + this.seconds;
        this.oldSeconds = 0f;
        this.startTime = Time.realtimeSinceStartup;
    }
    void Update()
    {
        float remainTime = this.totalTime - (Time.realtimeSinceStartup - this.startTime);
        if (remainTime <= 0f)
        {
            return;
        }

        float remainMinute = (int)remainTime/ 60;
        float remainSeconds = remainTime - remainMinute * 60;

        if ((int)remainSeconds != (int)oldSeconds)
        {
            this.timerText.text = String.Format("limit {0}：{1}", remainMinute.ToString("00"), ((int)remainSeconds).ToString("00"));
        }
        this.oldSeconds = remainSeconds;
        if (remainTime <= 0f)
        {
            Debug.Log("time finished");
        }
    }
}