Hi there ijcos,
You can rehearse your presentation in Keynote on Mac:
If you plan to use an external display to show your presentation, you can use rehearse slideshow mode to set up the presenter display in advance, and to practice your presentation, without having the external display connected.
Note: If you already have the external display connected, simply play the presentation to rehearse.
1. Open the presentation, then click to select the first slide in the slide navigator.
2. Choose Play > Rehearse Slideshow (from the Play menu at the top of your screen).
3. The presenter display appears.
Learn more here: Rehearse a Keynote presentation on your Mac
Also, you can add a clock or timer to the presenter display. Have a look at the steps in the 'Customize the presenter display' section of this link: Play a presentation on a separate display in Keynote on Mac
Take care!