js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>