I think now that the issue is clearer, I'll propose another solution.
The challenge now is that some of your files save the RecordingDate_Time as text (if the day is >12), while others store it as a datenum (if the day is <=12). That means there isn't a single import option you can use that will work with all your files.
Since you are in a time crunch to start your analysis, I'd propose a new approach. Load your table letting MATLAB decide how to read in that variable, and then convert that column to the correct datetime based on the datatype. Here, I wrote a function called formatTbl for that purpose.
I have used a month format so that there is no confusion between day and month. Feel free to update it. This is not extensively tested. I leave it to you to validate the results.
opts = detectImportOptions("P50-DE-test-2w...-IPG7-e3.xlsx","Sheet","NRT Peaks","TextType","string");
T1 = readtable("P50-DE-test-2w...-IPG7-e3.xlsx",opts);
Warning: Column headers from the file were modified to make them valid MATLAB identifiers before creating variable names for the table. The original column headers are saved in the VariableDescriptions property.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.
opts = detectImportOptions("P50-DE-test-6m...-IPG7-e3.xlsx","Sheet","NRT Peaks","TextType","string");
T2 = readtable("P50-DE-test-6m...-IPG7-e3.xlsx",opts);
Warning: Column headers from the file were modified to make them valid MATLAB identifiers before creating variable names for the table. The original column headers are saved in the VariableDescriptions property.
Set 'VariableNamingRule' to 'preserve' to use the original column headers as table variable names.
function Tout = formatTbl(Tin)
if strcmp(class(Tin.RecordingDate_Time),"string")
Tin.RecordingDate_Time = datetime(Tin.RecordingDate_Time,"InputFormat","d/M/uuuu H:mm:s","Format","d/MMM/uuuu h:mm:ss aa");
elseif strcmp(class(Tin.RecordingDate_Time),"datetime")
[y,M,d] = ymd(Tin.RecordingDate_Time);
[h,m,s] = hms(Tin.RecordingDate_Time);
Tin.RecordingDate_Time = datetime(y,d,M,h,m,s,"Format","d/MMM/uuuu h:mm:ss aa");