to create a date/time you can use the function DATE().
I suggest you enter the month number (rather than the name). Assuming you do this, you may follow the steps below.
Also... I live in the US where the decimal separator is the period ('.') and the thousand separator is the comma (','). I notice you are in Europe and the formulas you posted use the semicolon as the argument separator. replace the commas you see in my formulas with the semicolon.
the function DATE() takes threes arguments as follows:
DATE(YYYY, MM, DD)
you'll need to replace DATE with the word in Italian (I think)
for cell B3 you can use this formula as follows:
=DATE("Mese e anno"::$A$2, "Mese e anno"::$A$1, A3)
shorthand for this is:
B3=DATE("Mese e anno"::$A$2, "Mese e anno"::$A$1, A3)
select cel C3, copy select cells C3 thru the end of the column, paste