Hello and thank you for your answer.
I am not sure it responds to my confusion 
I indeed have a format problem that I have to correct from the spreadsheet - btw the integers in the first column are due to an import bug when I save the spreadsheet as a CVS from open office, without this extra column the CVS wouldn’t import properly in UE.
But I do not understand how (the first part of your answer) allows to automatically find the date of the day (today) and output the temperature data that corresponds to its row?
What I want to do is really this:
If (‘Date_1938_from_data_table’ = ‘today_date’) {
then output corresponding row for ‘Temp_Hamamatsu’
}
For exemple :
if (‘1 mar (1938)’ = ‘1 mar (20XX)’) {
then output 8.5
}
The idea is to have a system that updates automatically everyday.
Everyday, the corresponding date is found and the associated temperature is outputed.
(Then, I do the substraction between the 1938_temperature and today_temperature. But I figured this operation already.)
What I don’t get, is how to get the data I want from the data table everyday?
That is why in my screenshot I used the function ‘Now’ and ‘As date’ to output a string for today.
Then, the idea is to find the corresponding string within the table data and output the corresponding temperature.
So I understand that the date format from my CVS and Unreal date format have to be the same.
I just don’t know if that is the right way to do and also how to do it properly.
A far as I understand your answer doesn’t take in account this everyday-update aspect? or am I missing (likely) something?