怎么用javascript实现定时

<script language="javascript">var time_now_server,time_now_client,time_end,time_server_client,timerID;time_end=new Date("Jan 20, 2002 00:00:00");
time_end=time_end.getTime();time_now_server=new Date("Dec 21, 2001 13:11:1");
time_now_server=time_now_server.getTime();time_now_client=new Date();
time_now_client=time_now_client.getTime();time_server_client=time_now_server-time_now_client;setTimeout("show_time()",1000);function show_time()
{
timer.innerHTML =time_server_client; var time_now,time_distance,str_time;
var int_day,int_hour,int_minute,int_second;
var time_now=new Date();
time_now=time_now.getTime()+time_server_client;
time_distance=time_end-time_now;
if(time_distance>0)
{
int_day=Math.floor(time_distance/86400000)
time_distance-=int_day*86400000;
int_hour=Math.floor(time_distance/3600000)
time_distance-=int_hour*3600000;
int_minute=Math.floor(time_distance/60000)
time_distance-=int_minute*60000;
int_second=Math.floor(time_distance/1000)

if(int_hour<10)
int_hour="0"+int_hour;
if(int_minute<10)
int_minute="0"+int_minute;
if(int_second<10)
int_second="0"+int_second;
str_time="<font style='font-size:14pt;color:#28C84A'>"+int_day+"</font><font face='黑体' style='font-size:13pt;'>天</font><br>"+int_hour+":"+int_minute+":"+int_second;
timer.innerHTML=str_time;
setTimeout("show_time()",1000);
}
else
{
timer.innerHTML ="over";
clearTimeout(timerID)
}
}
</script>