Filed away for if/when I ever do another presentation: Presentation Clock ($0.99 for iPhone and iPad):

This app does one thing but it does it extremely well. If you give presentations, training, tutorials, or speak publicly, this app is for you. Beautifully animated, with large, easy to read numbers that change colors at thresholds you determine. When the timer hits 0:00, the colors invert (black on red) and continue counting up indicating how long you’ve gone over.

Reviews look good in the iTunes Store. Apps that focus on doing one thing really well are an easy $0.99 pick up for most folks.