Slider RT question

Hi, I have a trial where images appear for 4 sec and the slider component appears after that. I want to analyse reaction times for each slider click. Does anyone know how these reaction times are measured? Do they measure time between slider onset and the click? Or do they measure time between trial onset (when slider is not yet visible on the screen, as it only appears on sec 4) and click? Because it looks like the second is truth.