Workflow task due dates

Hi

I'm trying to create workflow tasks based on a Case being set to closed. I have the tasks creating OK; but I want their Due Date to be based on a number of days after the case is closed.

I've tried setting the Due Date to CARRule trigger date plus n days and to WF Rule Trigger Date plus n days. The tasks are created fine whenever I close a case; but the Due Date is always set to today and not to today + n days.

How do I set the Due Date into the future?

Thanks