Change Listing Expiry Option from Days to Months

I have customers who apparently are illiterate and do not understand how many months are in 180 days. So, I am needing to request or know how to change Days to Months for the package expiration options. Less headache and emails for me to deal with.

So if anyone knows how to do this WITHOUT modifying the code *to avoid doing again after an upgrade to another version) I would greatly appreciate it.

Thanks.