Clocks and timers for PowerPoint
There's a timer for PowerPoint 2000 and above from Tushar Mehta
There's also a free AutoDateTime addin available at PowerPoint MVP Chirag Dalal's site
And a free Break Timer from PowerPoint MVP, Bill Foley
Or a way of doing the job with animation, courtesy of Geetesh Bajaj
Kees van der Meer sent us this very clever countdown timer that can be used on any slide and uses only animation to do its work (requires PowerPoint 2002 or later). Download it, run the slide show, then look at the animation settings to see how it's done.
A clever "one-slide" timer from Andrew May at Microsoft.
PowerPoint MVP John Wilson takes Andrew's method and automates it with a little VBA. The VBA is only used to create the duplicate animated shapes, and isn't required in the actual presentation, which will work even in the VBA-less Viewer.
And here's John's Animated GIF timer.
With a bit of VBA, you can Simulate a countdown timer using Sleep API thanks to Shyam Pillai.