Relative reminder dates
How do I set a reminder that is relative to the date the task is due, and not to an absolute date. For instance, i have a task due April 10th and I want to know about it 2 days prior as a reminder. I can set the reminder for April 8. But if I move the due date for the task the reminder date doesn't move relative to it. :/
So how do I tell the system "Set the reminder X far ahead of the due date"?