[Leandro Sarmento da silva]

Helped me a lot!
I made some modifications to this expression, to make it the most common AE FX.
As an example, add a “Slider Control” on the key frame.
example:

effect (“Slider Control”) (“Slider”) added in place of the value of “beginCount” and “endCount”

beginCount = effect (“Slider Control”) (“Slider”);
endCount = effect (“Slider Control”) (“Slider”);

So you have control over the length “dur”
What you can put as “0”

For only with such modifications you already have the control when it starts and ends when through the key frame.