js 实现页面时间与当前时间同步

function showtime()
{
var today;
var hour;
var second;
var minute;
var year;
var month;
var date;
var strDate;
today=new Date();
var n_day = today.getDay();
switch (n_day)
{
case 0:{
strDate = "星期日"
}break;
case 1:{
strDate = "星期一"
}break;
case 2:{
strDate ="星期二"
}break;
case 3:{
strDate = "星期三"
}break;
case 4:{
strDate = "星期四"
}break;
case 5:{
strDate = "星期五"
}break;
case 6:{
strDate = "星期六"
}break;
case 7:{
strDate = "星期日"
}break;
}
year = today.getYear();
month = today.getMonth()+1;
date = today.getDate();
hour = today.getHours();
minute =today.getMinutes();
second = today.getSeconds();
if(month<10) month="0"+month;
if(date<10) date="0"+date;
if(hour<10) hour="0"+hour;
if(minute<10) minute="0"+minute;
if(second<10) second="0"+second;
document.getElementById('clock').innerHTML = year + "年" + month + "月" + date + "日 " + strDate +" " + hour + ":" + minute + ":" + second;
setTimeout("showtime();", 1000);
}