This question was raised in the blog about The Waters Rising getting a nomination this year when it was published in the US 2 years ago.  Here's what I said there:

The Clarke uses the UK publication year for the award which is why The Waters Rising got the nom this year. I updated the publication date on the novel page to show the 2011 Gollancz pub date. Just last year Tim Powers' book D eclare got a Clarke nomination when Corvus put it out in the UK even though it was originally published in the US in 2001 and won the WFA and was nominated for the Nebula and Locus that same year!

I always thought that must have been a nice surprise for Mr. Powers!

Here's something funny:  You'll note I typed "D eclare" and not "Declare".  It turns out that "declare" is a reserved word in SQL code.  Whenever we try to use it in the search or in the blog comments our servers think its a SQL injection attack!  They freak out and redirect to a maintenance screen.  You can't search WWEnd for Declare.  You have to search for Tim Powers instead to find it.