I have created a sample RadGrid web site using the provided code sample. On my side the sorting works as expected and takes into account the year too. Could you please ensure that your DataField's type is System.DateTime
and not System.String?
Please check out the attached application and try to distinguish the crucial differences between our projects.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed