How to prevent system convert subledger transaction to reporting ledger immediately event if the cur

Anybody Help?

Create new Subledger currency conversion level reporting ledger and the first open period is Aug-13, First Future Conversion Period is Oct-13. Period Aug-13 and Sep-13 open for both Primary Ledger and Reporting Ledger. But I don't want system convert Purchase Invoice which entry in period Sep-13 to Reporting Ledger. I would like to convert all of Payables Transaction and Receivables Transaction on period Oct-13. How to do like that? 

More information the first period of the Primary Ledger is Jan-12. the current period is Sep-13

First open period for Reporting ledger is Aug-13, and the current period is Sep-13 But The first Future Conversion Period is Oct-13.