How can i make count-down timer in ASP.NET AJAX?

I have a web-based test program and need a count - down timer

I don't think there is anything readily re-usable within ASP.NET Ajax for this. Just Google javascript countdown timer. There are loads of scripts around for this.http://www.google.co.uk/search?hl=en&safe=off&sa=X&oi=spell&resnum=0&ct=result&cd=1&q=javascript+countdown+timer&spell=1

I found a count-down timer .I think it's useful

http://www.codehouse.com/javascript/scripts/cool_countdown/