I shared the idea of using a YouTube Video as a Countdown Timer. I totally dropped the ball on this post and forgot to share that you have some options when inserting a video from YouTube or Google Drive.
Relevant to the timers in Google Slides is the fact that you can set a start and end time for the video. This means you can use the same countdown timer on multiple slides. If I insert a 20-minute video on a slide and want a 10-minute timer on the next slide, I can use the same countdown video and set the time to start at the 10-minute mark on the next slide using the "Video options..." in slides.
Here is a quick overview of the process.
1. In the Google Slides Insert menu. Click on the Insert Menu and then on Video.
2. Search for the length countdown timer you need.
3. Insert and resize the video and then right click on the video and choose click on "Video options..."
4. A sidebar will open and you can set the start time.