how go get datepart from FetchXML

Using FetchXML I would like to get a distinct list of years that Contacts were created on.

I can retrieve a the CreatedOn field, but what I want to retrieve the datepart year for CreatedOn.  Anybody know how I can do this?

<fetch version="1.0" output-format="xml-platform" mapping="logical" distinct="true"><entity name="contact"><attribute name="createdon" /></entity></fetch>