If your final goal is to extract the information from these 2 files with the same date and time, use function innerjoin.
For the second txt file, you may add Variable Name to each column since there is no header in this file.
% Read the first file
data1 = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1407529/Smoothing_DAY1_120s.txt','VariableNamingRule','preserve');
DateTime1 = datetime(data1.Date,'InputFormat','yyyy/MM/dd') + data1.Time;
data1.Date = DateTime1; % Update the datetime for each row and put in the 2nd column
data1.Time=[]; % Remove the unnecessary column
% Read the second file
data2 = readtable('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1407534/AB%20KH%20UKUR1_D1.txt');
DateTime2 = datetime(string(data2.Var3),'InputFormat','yyyyMMdd')+data2.Var4;
data2.Var3 = DateTime2; % Update the datetime for each row and put in the 3rd column
data2.Var4=[]; % Remove the unnecessary column
data2.Properties.VariableNames{3}='Date'; % Give the Variable Name to the the datetime column
T = innerjoin(data1,data2) % Use function innerjoin
