Why can't I use system variable building blocks in a user variable?

Using FrameMaker 11... I'd like to make a short variable that simply includes the full year -- no month, no date (e.g., "2016").  It is for a copyright page in my documents, that I'd just as soon forget about.  I just want to ensure that the year on this page is current (and auto-updates, if necessary when I build the book). 

I just had to re-PDF and re-release a document because of this (and our release process is anything but "speedy").