How to know Change had been made in OC41

Hi All

I have a quiry regarding T.Code OC41.Exchange rate.

If somebody has changed the rate of of one currency in any particular date, how can we know the user id, who has changed this currency rate??we have tried in utilities----change log, but there we could not find out the date which is appearing in OC41.

Please suggest

Thanks in Advance

Nikhil