Yes, you need to use DATEADD to convert your (fiscal?) date back to a calendar date, converting Day 1 of your year to be 1 Jan of the same calendar year. In your case it might be as simple as DATEADD('week',-14,[DateField])
Check this thread asking about something similar: Derive Fiscal Week/Month/Date from Calendar Date
The way you are doing it is wrong I guess. It is a in built functionality of the tool.
Follow this steps.
Then also change the database date properties by following this one.
I have kept week start as Tuesday since its 1st April 2014.
Let me know if this one helps.
Attaching the workbook for the same in the next post since having some trouble with internet.
Thanks and Regards,