This took me a while to work out what the issue was.
Basically I had another script that I've programmed as a events submission form that registered users can use. The issue was that the year was not displaying in jQuery date UI, until I looked at the source of the website I was unaware that mailchimp brings in it's own jQuery that overwrites any other jQuery. I haven't found a solution as of yet other than killing the mailchimp plugin while I work on the site. This is very frustrating!