It's very simple!

Since version 8.5.2, you can simply add an "Image" block and select the "Countdown" tool:

 

You will then be able to choose options such as the target date of the countdown, its appearance, its colors...

 

Click OK and it's done!

Your message has a personalized countdown timer that will update automatically!

The countdown is embedded in the message as an animated GIF.
Most email clients can read this format, but some will only display the first image of the animation.
However, in this case, this image will be updated automatically at each loading, so the animation may not be present but the countdown will be valid!