Counter

I am looking for some help on placing a timer on a PDF that will count down from 30 seconds. I tried using the app.setTimeOut() method to delay my counter function in order to simulate a timer. It does not count down completely to zero. Any suggestions. The code is below.

function counter()
{

if (milisec<=0)
{
milisec=9
seconds-=1
}

if (seconds<=-1)
{
milisec=0
seconds+=1
}
else
{
milisec-=1
clock.value=seconds+"."+milisec
app.setTimeOut("counter()",100)
}

}