Exchange rate during MIRO

Hi SAP Guru and Expert,

Currently user is having an issue about the exchange rate issue and not sure is it working and need your advise on it.

If on 12.12.2013 the exchange rate from USD to AUD is 3.44

and on 13.12.2013 the exchange rate from USD to AUD is 3.56.

When the invoice posting date is posted on 12.12.2013 the day itself the system will pick up 3.44 exchange rate.

On the day 13.12.2013, the invoice posting date is set to 12.12.2013, will the system pick up the exchange rate of 3.44 or 3.56.

Kindly advise and consult.

Thank you

Regards

Leonard Tan