lookup rate based on reported value

Hello Experts,

I have a new supplement where users should be able to select a month for which some specific accounts should be translated at that months close rate.

So during reporting of 2010.MAY the user might select that the accounts should be translated at 2010.FEB Close rate. In the time dimensions i have one property TIMEID so i can send in the value 20100200 on one account.

But my problem is that i don't know how to be able to make the lookup collect the end rate for 2010.FEB based on the value 20100200?

If you know how that can be done or have any idea for another solution it would be very appriciated.

Thankyou and Best regards,

Jonas