how to derive current week and previous week based on key date

Hi Gurus,

I have an urgent requirement in my current project as below

I am building a webi report based on bex query(By using BICS Connection), in my prompts i have key date it is not object it is a variable in bex query

my requirement is if user enter date it should show week number  and year.

Example:

If user enter keydate 12/31/2013 it should show 52.2013 and in another blank cell 51.2013.

Any suggestions would be appreciated.

thanks in advance.