var h1 =  document.querySelector('.hours')
     var m1 =  document.querySelector('.minute')
     var s1 =  document.querySelector('.second')
     var time = '2021-12-10 2:00:00'
    
     function timeS(){
      var nowTime = +new Date()  //获取此刻的时间 
    var inputTime = +new Date(time)  //输入的时间
    times = (inputTime - nowTime) / 1000    //获取剩余的总毫秒数    剩余的总秒数 = 输入的时间毫秒数 - 此刻的时间毫秒数
    h = parseInt( times/ 60 /60 %24) >=10 ? parseInt( times/ 60 /60 %24) : "0" + parseInt( times/ 60 /60 %24)  //小时
    m =parseInt( times / 60 % 60 )>10 ? parseInt( times / 60 % 60 ) : "0" + parseInt( times / 60 % 60 )  //分钟
    s =parseInt( times % 60) > 10? parseInt( times % 60) : "0" + parseInt( times % 60)
    h1.innerHTML  = h
    m1.innerHTML = m
    s1.innerHTML = s
  } 
  timeS()
  setInterval(timeS,1000)

  
    <div class="hours"></div>
    <div class="minute"></div>
    <div class="second"></div>

 div{
    width: 50px;
    height: 50px;
    background-color: blue;
    margin: 0 0 0 10px;
    float: left;
    color : white;
    text-align: center;
    line-height: 50px;
  }