How to extract/format the calendar week from date?

Hi,

we are using WebI XI 3.1 SP3 in combination with SAP-BI(BW) 7.2.

From a BW-Query we get a normal date (21.03.2011) and we want

to show our KPIs/ key data per calender week (12.2011).

In the universe-designer we only can get calender month from a calender day

(right click to the dimension --> object format --> Date/.. --> Mmm jjjj).

So the question is, HOW CAN WE GET THE CAL WEEK within SAP-BO (Universe/WEBI)?

Thanks for help.