@DABU
Note that although both these pages refer to the annual currency exchange rate, both pages also say the transactions must be recognized using the exchange rate on the date of the transaction.
https://www.irs.gov/individuals/international-taxpayers/yearly-average-currency-exchange-rates
https://www.irs.gov/individuals/international-taxpayers/foreign-currency-and-currency-exchange-rates
Note that the IRS will generally accept any currency conversion rate that is “reasonable“. If the exchange rate fluctuates during the year so that reporting using the average rate gives you significantly less US income than reporting the daily rate, the IRS may not view the annual rate as reasonable in your situation.
