This is from the IRS website. It looks like you can use any posted exchange rate for the year.
Exchange Rates
1. When converting foreign currency to U.S. currency for purposes
of filing a U.S. tax return, what foreign currency exchange rate should
I use?
The Internal Revenue Service has no official exchange rate.
Generally, it accepts any posted exchange rate that is used
consistently.
If you have a single transaction, such as the sale of a business that
occurred on a single day, use the exchange rate for that day. However,
if you receive income evenly throughout the tax year, you may translate
the foreign currency to U.S. dollars using the yearly average currency
exchange rate for the tax year.
If there is more than one exchange rate, use the one that applies to
the facts and circumstances on a consistent basis. Refer to the Foreign Currency and Currency Exchange Rates web page.
