Countdown Clock Help/Query if possible?

Hello,

I did some searching and couldn’t find anything to what I’m about to describe. I was wondering if anyone knew of a Countdown Clock Script for 20 minutes. I’m working on a game for an elementary class, and each segment of the game is 20 minutes long. Is there a way to make a countdown like this? And how would I apply the script to an instance (text field)?

Or if anyone has a link or file that can teach me this I would be very thankful. I am using Flash 8.

Richard