SOURCE

$(function() {
  /*当前时间*/
  var t = null;
  function clock() {
    var _date = new Date();
    var _y = _date.getFullYear();
    var _m = _date.getMonth() + 1;
    var _d = _date.getDate();
    var _h = checkTime(_date.getHours());
    var _ms = checkTime(_date.getMinutes());
    var _s = checkTime(_date.getSeconds());
    document.getElementById('date').innerHTML = '当前时间 : ' + _y + '-' + _m + "-" + _d + '&nbsp' + _h + ':' + _ms + ':' + _s;
    t = setTimeout(clock, 1000);
  }
  clock();
}) function countdown(et) {
  var _start = new Date();
  var _end = new Date(et);
  var _countdown = ((_end.getTime() - _start.getTime()) / 1000);
  var _d = parseInt(_countdown / (60 * 60 * 24));
  var _h = parseInt(_countdown / (60 * 60) % 24);
  var _m = parseInt(_countdown / 60 % 60);
  var _s = parseInt(_countdown % 60);
  document.getElementById('countdown').innerHTML = '距离活动开始还有' + checkTime(_d) + '天' + checkTime(_h) + '小时' + checkTime(_m) + '分' + checkTime(_s) + '秒';
  ts = setTimeout(function() {
    countdown('2020/7/18 24:00:00')
  },
  1000);
}
function checkTime(i) {
  return i < 10 ? i = '0' + i: i;
}
window.onload = function() {
  var ts = null;
  countdown('2020/7/18 24:00:00');
}

<div id="date"></div>
  <div id="countdown"></div>