No year of publication for biblioentry

I'm converting a set of papers - about 60 in total - from a custom XML
format (we used whatever tags we thought appropriate) to DocBook.

It's been a long and sometimes painful process, but the result is
worth the effort.

I have a couple of problems, but I'll try to address them one by one
when I'm sure I can't find the solution via other means.

Here's the first:
There's no year of publication element in a biblioentry. Here's a
sample biblioentry:

  <biblioentry>
   <title>Interface morphology development during stress corrosion
cracking: Part I, via surface diffusion.</title>
   <authorgroup>
     <author><firstname>R.</firstname><surname>Asaro</surname></author>
     <author><firstname>W.</firstname><surname>Tiller</surname></author>
   </authorgroup>
   <pagenums>1789-1796</pagenums>
  </biblioentry>

The year the book was published is very important, as later additions
may not include the correct pages. I can see this being especially
important when citing online work.

A simple <year> tag would be fine, I think. Unless there's a good
reason to add a new element, <yearofpub>, however I think it's
unwarranted.

In the book, I see something like: <copyright><year>1997</year> ..
other info</copyright>. Although I could possibly use this, it's
hardly accurate.

Could this be added into the spec? Is there a workaround? Is there any
reason I shouldn't be doing this?

Kind Regards,
Aidan