javascript计算用户打开网页的停留时间

<SCRIPT LANGUAGE="JAVASCRIPT">
<!--
var ap_name = navigator.appName;
var ap_vinfo = navigator.appVersion;
var ap_ver = parseFloat(ap_vinfo.substring(0,ap_vinfo.indexOf('(')));
var time_start = new Date();
var clock_start = time_start.getTime();
var dl_ok=false;
function init ()
{
if(ap_name=="Netscape" && ap_ver>=3.0)
dl_ok=true; 
return true;
}
function get_time_spent ()
{ 
var time_now = new Date();
return((time_now.getTime() - clock_start)/1000); 
}
function show_secs () // show the time user spent on the side
{ 
var i_total_secs = Math.round(get_time_spent()); 
var i_secs_spent = i_total_secs % 60;
var i_mins_spent = Math.round((i_total_secs-30)/60); 
var s_secs_spent = "" + ((i_secs_spent>9) ? i_secs_spent : "0" + i_secs_spent);
var s_mins_spent ="" + ((i_mins_spent>9) ? i_mins_spent : "0" + i_mins_spent);
document.fm0.time_spent.value = s_mins_spent + ":" + s_secs_spent;
window.setTimeout('show_secs()',1000); 
}
// -->
</SCRIPT>