Hi Aaron,
I don’t see an easy way to add the year to the Post List Widget. The best way I could think to do it would be to hook a meta tag into the blog page template using a plugin like Code Snippets.
Could you please send us the URL of a page on your site where you have this functionality added? This will allow us to take look into your issue more directly and possibly offer you some better suggestions on how to go about adding your tag.
We look forward to hearing from you!