Countdown

I assume you mean a number on the screen that goes down every second to zero until the end of the trial?

Short answer: Not it is not easy to implement

You would typically not use that in a cognitive psychological experiment because it distracts from the cognitive process you try to study. This because people will "attend" the countdown and because it will "interfere" with other cognitive processes. I would recommend against it, but it depends on what you try to do.

Note that in PsyToolkit SURVEYS, you can have a countdown on the screen! See link below for the documentation:

www.psytoolkit.org/doc2.5.0/online-survey-syntax.html#maxtime