Gysbert is correct and you may trust that with his experience he is very unlikely to get something like this wrong. Did you try his solution?
To prove to yourself what is happening simply put your Month_year column in a table and then put "=Date(Month_name -13)" in another column. Then try a third column with just "=Month_name-13". You will notice that while your Month_name column is being presented as a combination of month and year, it is in fact stored as a dual (you can confirm that by checking the help on the monthname function) with the full date recorded in number form, so it is a date as Gysbert has said.