Trying to get onset time of first keypress using a sliding scale in Psychtoolbox

I have a slider which you can move left and right and I am trying to get the onset time of the first keypress. However, right now I only get the time for how long the rating scale is on the screen. Any suggestions?

Thanks in advance!

% Loop for scale loop
t0                         = GetSecs;
answer                     = 0;
moved                      = 0;
firsttouch                 = 0;
while answer == 0
    % Parse user input for x location
    if strcmp(device, 'mouse')
        [x,~,buttons,~,~,~] = GetMouse(screenPointer, 1);
    elseif strcmp(device, 'keyboard')
        [~, secsrating, keyCode, ~] = KbCheck;
        if keyCode(responseKeys(2)) == 1 && firsttouch == 0
            x = x - stepSize; % Goes stepSize pixel to the left
            onset_rating = secsrating-t0;
            firsttouch = 1;
        elseif keyCode(responseKeys(2)) == 1 && firsttouch == 1
            x = x - stepSize; % Goes stepSize pixel to the left
            onset_rating=0;
        elseif keyCode(responseKeys(3)) == 1 && firsttouch == 0
            x = x + stepSize; % Goes stepSize pixel to the right    
            onset_rating = secsrating-t0;
            firsttouch = 1;
        elseif keyCode(responseKeys(3)) == 1 && firsttouch == 1
            x = x + stepSize; % Goes stepSize pixel to the right    
            onset_rating=0;
        end
    else
        error('Unknown device');
    end