Relative due dates

Hi, I’m trying to set up a project where I know that (for example) a specific task is due 18 weeks before the project due date. Unfortunately the -18w option seem to be relative to today rather that the due date of the project. Is there a way around this?

On a similar note, is there any way to make the due date relative to the defer date (i.e. +1W would mean 1 week after defer date, not 1 week from today)