How to create count-down timer in javascript

<!-- copy and paste into your editor -->
<!-- it works in latest version of crome and  mozilla-->
<form name="count">

Select day for Count Down
<input type="date" onchange="countdown()" name="sdate" id="sdate" />
<p id="count3"></p>
</form>


<script>



var current="today"
var montharray=new Array("Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec")

function countdown(yr,m,d)
{
var select_day=document.getElementById("sdate").value;

var n=select_day.split("-");
yr=n[0];m=n[1];d=n[2];

theyear=yr;themonth=m;theday=d;
var today=new Date();
var todayy=today.getYear();

if (todayy < 1000)
{
todayy+=1900
var todaym=today.getMonth()
var todayd=today.getDate()
var todayh=today.getHours()
var todaymin=today.getMinutes()
var todaysec=today.getSeconds()
var todaystring=montharray[todaym]+" "+todayd+", "+todayy+" "+todayh+":"+todaymin+":"+todaysec;

futurestring=montharray[m-1]+" "+d+", "+yr
dd=Date.parse(futurestring)-Date.parse(todaystring)
msPerDay = 24 * 60 * 60 * 1000 ;
dday=Math.floor(dd/(msPerDay))
dhour=Math.floor((dd%(msPerDay))/(60*60*1000)*1)
dmin=Math.floor(((dd%(msPerDay))%(60*60*1000))/(60*1000)*1)
dsec=Math.floor((((dd%(msPerDay))%(60*60*1000))%(60*1000))/1000*1)

if(dday==0&&dhour==0&&dmin==0&&dsec==1)
{
document.forms.count.count2.value=current
return
}

else
{
//document.forms.count.count2.value="Only "+dday+ " days, "+dhour+" hours, "+dmin+" minutes, and "+dsec+" seconds";
document.getElementById("count3").innerHTML= dday+" days "+dhour+" :"+dmin+" :"+dsec;
setTimeout("countdown(theyear,themonth,theday)",1000)
}
}

}
//countdown(2013,12,25)
//2013-05-27

</script>