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You can enter the actual exchange rates for all currencies with respect to the national currency on the Money > Master Data > Currencies form by using the Currency Rates link on the top of the form. For more details, see Adding an Exchange Rate.
In case when accounting currency is not the national currency, you will need to determine the exchange rate of a transaction currency with respect to the accounting currency based on two other exchange rates:
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To include this calculation in the month-end closing process, you can select the Calculate Exchange Rate Differences option on the Accounting > Service Tools > Month-End Closing form. You can review the results of the calculation and corresponding journal entries in the generated Month-End Closing document when you click the 
button. For more information on month closing, refer to Month-End Closing Service Tool.
During month-end closing process, balances of the cash accounts that are maintained in foreign currencies are revalued in the accounting currency by using the exchange rates on the date of the month closing and compared to their balances in the accounting currency on the last revaluation date - any differences are recorded as exchange rate differences.
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If the resulting sum is equal or less than -0.005, the sum is posted to the default account selected in the Losses on Exchange Differences column on the Accounting > See Also > GL Accounts by Default form.
(переоцениваются ли налоговые счета в валюте учета?)