As a timer, one way to notify online speakers of their times is to use Zoom Virtual Backgrounds.
You can download the above backgrounds at this link.
Here’s a good video of the process.
As Timer, you may want to remind online speakers to PIN or move your Timer video to the top left position so they can easily keep track of the time.
You can use the background below as your first background so that online speakers know who is the designated online Timer. You can also download and use the red, yellow and green pictures below.