Does the Timer work with Zoom Whiteboards?

Yes! See below for an example. You'll need to select "My Video" as the display method. From there, you'll probably want to select "split screen," "only timer," or "my video in corner." Using the "timer in corner" option will probably be too small for participants to see.

To adjust the relative sizes of the whiteboard vs. participant video, click and drag the center bar separating the two sides, as shown in the red circle below.