So, I answered my own question. But, maybe someone has a better solution!
I take the difference between the times of the measurements and remove all of the positive durations. I do this because you will always need the previous reflectivity measurement. Then i find the minimum of the absolute value of the duration and save the index in a new array.
x = refTT.('Time'); % this is unnecessary but I initialize arrays with the datetimes
y = [rawDataTT.('Var1')];
for n = 1:height(refTT) % take the time diff for each rawDataTT row
time_diff(n,:) = x(n,1)-y(:,1);
end
time_diff(time_diff > 0) = NaN; % remove positive durations
[~,I] = min(abs(time_diff(:,:)),[],'omitnan'); % find the minimum backwards duration
for k = 1:height(rawDataTT) % do the calculation with the index in I
extinction(k,:) = refTT.('Reflectivity')(I(k),:) + ext_bath .*...
(rawDataTT.('Spectra')(k,:);
end