There are ways to populate a dropdown, but it's really not worth pursuing for such a short list. You could have populated the list manually in the time it took to post this question. Once you have one such field created, it can be resused elsewhere by copy/paste and giving it a different field name.
Regarding the years, can you clarify what your question is? Do you want to use a dropdown for it as well?
And you are aware of the possible options for listing the months and having an optional export value for each month.
For listing the months, you can use a number, a 3 character string, or the full name.
I would look at deciding how you want to populate the drop down box and build a one page form with just this field. You can then insert that page into a PDF and copy the field as needed and then delete the inserted page. Or you could create an FDF file to create the drop down box.