ABAP Function module for BW

Hello Gurus,

I am not an ABAPer, I work in BW module.

I have a requirement to write a routine which will check the following logic

In BW, I have a field called Posting Date (0PSTNG_DATE). This field is currently set to sy-datum. But i need to write a routine which will find last friday's date and update this field with friday's date.

Eg: If sy-datum = 03/09/2007 then i need to populate posting date field as 31/08/2007.

Sy-datum can be any day of the week. But it should always write last week friday's date.

Is there any standard function module which i can use here.

Thanks in advance.

Cheers

POPS