/*
  定时器：
  setTimeout 只执行一次的定时器 
  clearTimeout 关闭只执行一次的定时器
  setInterval 反复执行的定时器
  clearInterval 关闭反复执行的定时器
*/
var time1 = setTimeout(myalert,2000);
var time2 = setInterval(myalert,2000);
/*
clearTimeout(time1);
clearInterval(time2);
*/
function myalert(){
  alert('ok!');
}

<script type="text/javascript">
  window.onload = function(){  
    var oDiv = document.getElementById('div1');
    function timego(){
      var now = new Date();
      var year = now.getFullYear();
      var month = now.getMonth()+1;
      var date = now.getDate();
      var week = now.getDay();
      var hour = now.getHours();
      var minute = now.getMinutes();
      var second = now.getSeconds();
      var str = '当前时间是：'+ year + '年'+month+'月'+date+'日 '+toweek(week)+' '+todou(hour)+':'+todou(minute)+':'+todou(second);
      oDiv.innerHTML = str;
    }
    timego();
    setInterval(timego,1000);
  }
  function toweek(n){
    if(n==0)
    {
      return '星期日';
    }
    else if(n==1)
    {
      return '星期一';
    }
    else if(n==2)
    {
      return '星期二';
    }
    else if(n==3)
    {
      return '星期三';
    }
    else if(n==4)
    {
      return '星期四';
    }
    else if(n==5)
    {
      return '星期五';
    }
    else
    {
      return '星期六';
    }
  }
  function todou(n){
    if(n<10)
    {
      return '0'+n;
    }
    else
    {
      return n;
    }
  }
</script>
......
<div id="div1"></div>

<script type="text/javascript">
  window.onload = function(){
    var oDiv = document.getElementById('div1');
    function timeleft(){
      var now = new Date();
      var future = new Date(2016,8,12,24,0,0);
      var lefts = parseInt((future-now)/1000);
      var day = parseInt(lefts/86400);
      var hour = parseInt(lefts%86400/3600);
      var min = parseInt(lefts%86400%3600/60);
      var sec = lefts%60;
      str = '距离2016年9月12日晚24点还剩下'+day+'天'+hour+'时'+min+'分'+sec+'秒';
      oDiv.innerHTML = str; 
    }
    timeleft();
    setInterval(timeleft,1000);    
  }
</script>
......
<div id="div1"></div>