在别的类中直接调用TimeManager.Instance.BeginTimer 

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

/// <summary>
/// 计时器管理器
/// </summary>
public class TimerManager : MonoBehaviour
{
    //单例
    private static TimerManager _instance;
    public static TimerManager Instance
    {
        get
        {
            if (_instance == null)
            {
                GameObject go = new GameObject(typeof(TimerManager).ToString());
                DontDestroyOnLoad(go);
                _instance = go.AddComponent<TimerManager>();

            }
            return _instance;
        }
    }

    public static bool isSync = true;//是否为同步执行

    [HideInInspector] public List<TimerData> timerList = new List<TimerData>();//倒计时列表(存储所有的倒计时)

    private float lastUpdateTime;//上一帧的时间(忽略时间缩放影响的真实时间)

    private void Awake()
    {
        lastUpdateTime = Time.realtimeSinceStartup;
    }

    /// <summary>
    /// 开始倒计时
    /// </summary>
    /// <param name="countdown">倒计时时间</param>
    /// <param name="onFinish">回调</param>
    /// <param name="timerName">计时器名称</param>
    /// <param name="ignoreTimeScale">是否忽略时间缩放</param>
    public void BeginTimer(float countdown, Action onFinish = null, string timerName = "", bool ignoreTimeScale = false)
    {
        TimerData timeData = new TimerData();
        timeData.SetTimer(countdown, onFinish, timerName, ignoreTimeScale);
        timerList.Add(timeData);
    }

    /// <summary>
    /// 清除倒计时
    /// </summary>
    /// <param name="timerName">计时器名称</param>
    public void ClearTimer(string timerName = "")
    {
        if (timerName == "")
        {
            timerList.Clear();
        }
        else
        {
            foreach (var timer in timerList)
            {
                if (timer.timerName == timerName)
                {
                    timerList.Remove(timer);
                    return;
                }
            }
        }
    }

    /// <summary>
    /// 得到倒计时时间
    /// </summary>
    /// <param name="timerName">计时器名称</param>
    public float GetTimer(string timerName)
    {
        foreach (var timer in timerList)
        {
            if (timer.timerName == timerName)
            {
                return timer.timer;
            }
        }
        Debug.LogWarning("没有创建倒计时：" + timerName);
        return -1;
    }

    private void Update()
    {
        //计算真实的时间增量
        var realTime = Time.realtimeSinceStartup - lastUpdateTime;
        lastUpdateTime = Time.realtimeSinceStartup;

        //如果是同步则一直遍历倒计时列表并同步计算
        if (isSync)
        {
            for (int i = 0; i < timerList.Count; i++)
            {
                if (timerList[i] == null)
                {
                    break;
                }

                timerList[i].UpdateTimer(timerList[i].ignoreTimeScale ? realTime : Time.deltaTime);
                if (timerList[i].isFinish)
                {
                    timerList.RemoveAt(i);
                    i--;
                }
            }
        }
        //如果是异步则只对列表中的第一个倒计时进行计算
        else
        {
            if (timerList.Count == 0 || timerList[0] == null)
            {
                return;
            }

            timerList[0].UpdateTimer(timerList[0].ignoreTimeScale ? realTime : Time.deltaTime);
            if (timerList[0].isFinish)
            {
                timerList.RemoveAt(0);
            }
        }
    }
}

/// <summary>
/// 每一个计时器的数据类
/// </summary>
public class TimerData
{
    public float timer;//计时器   
    public bool isFinish;//本次倒计时是否完成

    private Action onFinish;//回调  
    public string timerName;//计时器名称
    public bool ignoreTimeScale;//是否忽略时间缩放

    /// <summary>
    /// 设置计时器
    /// </summary>
    /// <param name="countdown">倒计时时间</param>
    /// <param name="callback">回调</param>
    /// <param name="timerName">计时器名称</param>
    /// <param name="ignoreTimeScale">是否忽略时间缩放</param>
    public void SetTimer(float countdown, Action onFinish, string timerName, bool ignoreTimeScale)
    {
        timer = countdown;
        this.onFinish = onFinish;
        this.timerName = timerName;
        this.ignoreTimeScale = ignoreTimeScale;
    }

    /// <summary>
    /// 更新计时器
    /// </summary>
    /// <param name="deltaTime">增量时间</param>
    public void UpdateTimer(float deltaTime)
    {
        timer -= deltaTime;
        if (timer <= 0)
        {
            isFinish = true;
            onFinish?.Invoke();
        }
    }
}