I had to guess that you wanted to exclude 20:01:01 itself.
I wonder if what you would really want is "after 6 am (excluding 6 am), up to and include 8 pm" ? Or maybe 6 am (inclusive) up to but excluding 8 pm ?
filename = 'https://www.mathworks.com/matlabcentral/answers/uploaded_files/880370/test%20data.xlsx';
opt = detectImportOptions(filename, 'readvariablenames', false);
opt = setvartype(opt, 1, 'datetime');
opt = setvaropts(opt, 1, 'InputFormat', 'uuuu-MM-dd HH:mm:ss');
T = readtable(filename, opt);
[h, m, s] = hms(T{:,1});
d = duration(h, m, s);
starttime = duration(06,01,01);
endtime = duration(20,01,1);
mask = isbetween(d, starttime, endtime, 'openright');
selected_T = T(mask,:);
selected_T(end-10:end,:)
