How can I a specific exchange rate date (End of month, typically) to create the accountings?

Notes for the case.

When I create a lease in the standard way and then I run the concurrent "Transfer Normalized Lines to GL" to create the accounting. If I run it again the process will create a reverse and a new accounting as of the sysdate.

In our case we are using a "foreing currency" that shows the inflation and every time we create the accounting use a different exchange rate based on the sysdate. My question is,

How can I configure the process in order to always have the exchange rate instead of the sysdate, select the last exchange rate of the period?