How To create Count Down Timer in Javascript?

Try This code to create count Down Timer in javascript.

Here counter is form name u can change it .
and timer is control where u display.
//Javascript

var milisec=0
var seconds=30
document.counter.timer.value='600'
function display(){
if (milisec<=0){
milisec=9
seconds-=1
}
if (seconds<=-1){
milisec=0
seconds+=1
}
else
milisec-=1
document.counter.timer.value=seconds+"."+milisec
setTimeout("display()",100)
}
display()
//End Javascript