调用的时候看你有哪些数据，然后调用不同的方法。
1、已有倒计时天，小时，分钟，秒调用startData方法
2、已有当前时间和目标时间调用initData
上面两个方法的参数有个callBack需要你传入一个方法，用来接收倒计时通知，会返回两个数据：

/**
 * js倒计时对象
 */
let countdownObj = {
  timer: null,
  syncFlag: false,
  d: '00',
  h: '00',
  i: '00',
  s: '00',
  leftTime: 0,
  seconds: 0,
  progressIsCallBack:true,
  callBackFun:null,
  /**
   * 时间需要处理，根据格林威治初始化
   * @param {*} callBack 结束时需要调用的方法
   * @param {*} endTime 目标时间
   * @param {*} serverDate 获取的服务器当前时间
   */
  initData(callBack,endTime,serverDate){
    let targetTime =  Date.parse(endTime);
    let serverTime = Date.parse(serverDate); // 请求服务端接口，返回服务器当前时间戳
    let localTime = this.getNowDate(8); // 用户本地时间戳
    let timeOff = serverTime - localTime;
    let rightTargetTime = targetTime - timeOff; // 去除偏差后的目标时间
    let newDate = this.getTimeDifference(serverDate,rightTargetTime);
    this.startData(callBack,newDate.day,newDate.hour,newDate.minute,newDate.second);
  },
  /**
   * 倒计时起始方法（传入天，小时，分钟，秒）
   * @param {*} callBack 结束时需要调用的方法
   * @param {*} day 倒计时天
   * @param {*} hour 倒计时小时
   * @param {*} minute 倒计时分钟
   * @param {*} second 倒计时秒
   * @param {*} timestamp 
   * @returns 
   */
  startData(callBack,day=0,hour=0,minute=0,second=0,timestamp = 0) {
    this.callBackFun = callBack;
    this.day = day;
    this.hour = hour;
    this.second = second;
    this.minute = minute;
    this.timestamp = timestamp;
    this.seconds = this.toSeconds(this.timestamp, this.day, this.hour, this.minute, this.second)
    if (this.seconds <= 0) {
      this.seconds = this.toSeconds(0, 0, 0, 0, 0)
      this.countDown()
      return
    }
    clearInterval(this.timer)
    this.countDown()
    this.timer = setInterval(() => {
      this.seconds--
      if (this.seconds < 0) {
        this.timeUp()
        return
      }
      this.countDown()
    }, 1000)
  },
  /**
   * 返回天时分钟秒
   * @param {*} startDateString 当前时间
   * @param {*} endDateString 目标时间
   * @returns 
   */
  getTimeDifference(startDateString, endDateString) {
      var start = new Date(startDateString); // 开始日期
      var end = new Date(endDateString); // 结束日期
      
      var timeDiffInMs = Math.abs(end - start); // 获取时间差（单位为毫秒）
    
      var diffDays = Math.floor(timeDiffInMs / (1000 * 3600 * 24)); // 计算天数
      var diffHours = Math.floor((timeDiffInMs % (1000 * 3600 * 24)) / (1000 * 3600)); // 计算小时数
      var diffMinutes = Math.floor((timeDiffInMs % (1000 * 3600)) / (1000 * 60)); // 计算分钟数
      var diffSeconds = Math.floor((timeDiffInMs % (1000 * 60)) / 1000); // 计算秒数
    
      return {
        day:diffDays,
        hour:diffHours,
        minute:diffMinutes,
        second:diffSeconds
      }
  },
 
  /**
   * 本地时间
   * @param {*} timeZone 
   * @returns 
   */
  getNowDate(timeZone) {
    var timezone = timeZone || 8; //目标时区时间，东八区
    // 本地时间和格林威治的时间差，单位为分钟
    var offset_GMT = new Date().getTimezoneOffset(); 
    // 本地时间距 1970 年 1 月 1 日午夜（GMT 时间）之间的毫秒数
    var nowDate = new Date().getTime(); 
    var targetDate = nowDate + offset_GMT * 60 * 1000 + timezone * 60 * 60 * 1000;
    return targetDate;
  },
  countDown() {
    let seconds = this.seconds
    let [day, hour, minute, second] = [0, 0, 0, 0]
    if (seconds > 0) {
      day = Math.floor(seconds / (60 * 60 * 24))
      hour = Math.floor(seconds / (60 * 60)) - (day * 24)
      minute = Math.floor(seconds / 60) - (day * 24 * 60) - (hour * 60)
      second = Math.floor(seconds) - (day * 24 * 60 * 60) - (hour * 60 * 60) - (minute * 60)
    } else {
      this.timeUp()
    }
    if (day < 10) {
      day = '0' + day
    }
    if (hour < 10) {
      hour = '0' + hour
    }
    if (minute < 10) {
      minute = '0' + minute
    }
    if (second < 10) {
      second = '0' + second
    }
    this.d = day
    this.h = hour
    this.i = minute
    this.s = second
    if(this.progressIsCallBack){
      this.callBackFun('timeprogress',{day,hour,minute,second})
    }
    
  },
  timeUp() {
    clearInterval(this.timer)
    this.callBackFun('timeup',{})
  },
  toSeconds(timestamp, day, hours, minutes, seconds) {
    if (timestamp) {
      return timestamp - parseInt(new Date().getTime() / 1000, 10)
    }
    return day * 60 * 60 * 24 + hours * 60 * 60 + minutes * 60 + seconds
  },
}