Countdown Script for Web - 250 down to 0

Need Script Fast - Could be Javascript or CGI, I don't know Need a countdown display updated every time someone fills out a form and gets sent to a "thankYou" page. This works like an inventory item counter. Example only 250 units available, after someone fills out any order form and gets to the Thank You page. The Form entry page now shows only "249" available. next time "248" and so on down to 0. So two HTML pages "communicate with each other" The one page sends a (minus 1) to the other page to reduce inventory number after successful form completion. The number of units to countdown must be changeable from 1 to 1000 on initial setup. Right now I need it to start at 230 and count down from there. Site is running on Windows Server Whatever type of script would be fastest and easiest to install. Need immediately. Thanks