You are supposed to translate at the transaction date:
"Make all income tax determinations in your functional currency. If your
functional currency is the U.S. dollar, you must immediately translate
into dollars all items of income, expense, etc. (including taxes), that
you receive, pay, or accrue in a foreign currency and that will affect
computation of your income tax. Use the exchange rate prevailing when
you receive, pay, or accrue the item. If there is more than one exchange
rate, use the one that most properly reflects your income. You can
generally get exchange rates from banks and U.S. Embassies."
Tom Young
