File operations writing and comapring

Dear Support,

I'd like to ask your help for file operations. 

I want to write a variable's value daily with date into a file (append each day with a row) and then after a time compare the latest value with an avarage value of specified timespan (for example a month/30 days).

The file should look like this:

2022-10-20,45667
2022-10-21,22344
...
...
2022-11-20,4432

so after a month passed (for example) I want to avarage the of the values compared to the last current one.

I tried file operation library but I couldn't get them working, even after reading the manual. I have problem accessing the variable's memory address (it's a retain one) and using the functions, function blocks to writing and reading back the file.