What-IF parameter to select rows for another What-IF parameter

Hi,

I have average handling time for 6 months which is projected for future dates (upcoming 12 months). I can manage to build parameter to change measure' value but it is changing all rows in the table. In ideal scenario, I would like to give an option to also select row/date in which parameter should be applied. My initial idea was to create index column in dimensional date table and build measure based on it.

Something like:

Measure = 
var index_ = max(dimdate[index_)
return if(index_ < parameter value, handling time + what-if parameter, handling time

However, it did not work out for me at all.

My desirble output is below:

I would appreciate any help. Thank you