The trick is that you embed a timer that has been turned into a YouTube video. YouTube is used because it can be embedded into most presentation platforms. Then you use a picture in picture technique to indicate time is running out with game show music and follow that up with a "time is up" sound effect.
Here's what it looks like.
I tested this last week when speaking to several hundred parents (picture below) interested in learning about the home-school connection. It worked great. Try it out in your own setting and comment to let us know how it goes.