You could do it like this, if you need it to go to 0. If you need it to match the opacity of another layer, replace the “0” by pick-whipping the opacity of that other layer.
if (marker.numKeys > 1 ){
if (time >= marker.key(1).time){
thisComp.layer("N_Op_text").transform.opacity.valueAtTime(time-(marker.key(1).time));
if (marker.numKeys == 2 && time >= marker.key(2).time){0};
}else{value}
}else{value}
(Will only work properly if you have two markers on the layer.)
Kalleheikki Kannisto
Senior Graphic Designer