You had numerous errors in your code. A key one is that you were writing the sensor data and time to the same column (B1). Since the time values were written after the sensor data this would overwrite the sensor data and only display time. You however had other problems so that you were writing anything at all, and so this overwrite problem didn't show up. Here is a cleaned up version of your idea that should be closer to what you want.
Note in particular that you don't need to be putting everything into cell arrays. Also since you are writing columns in Excel you should be careful to have the data and times in n by 1 arrays, with your code they were in 1 by n arrays. Also you need to be careful about being consistent with capitalization, you used both k and K for your counter, you have to pick one or the other and stay with it, MATLAB is case sensitive so k and K are assumed to be different variables.
% set some parameters just for testing/debugging
kMax = 100;
sensorvalue = randn(kMax,1);
% preallocate arrays
app.iwant = zeros(kMax,1)
app.iwant_time = NaT(kMax,1)
%start button
k=0
while true && k < kMax
% loop code
date=datetime("now","Format","HH:mm:ss");
k=k+1
app.iwant(k)=sensorvalue(k);
app.iwant_time(k) = date;
end
% convert datetime array to Excel datenumber
app.iwant_time = exceltime(app.iwant_time);
%save button, time goes in column A, values go in column B
xlswrite('test.xls',app.iwant,'Visits','B1');
xlswrite('test.xls',app.iwant_time,'Visits','A1');
