The rate your accountant already recognises.
The European Central Bank publishes euro foreign exchange reference rates for around thirty currencies. They come out of a daily concertation procedure between European central banks that normally takes place around 14:10 CET, and they land on the ECB site around 16:00 CET on every working day that is not a TARGET closing day. The rates are free, need no API key, and are served both as a daily XML feed and through the ECB Data Portal web service, where a key like D.USD.EUR.SP00.A returns the daily dollar rate against the euro.
EU VAT rules let you convert a foreign-currency invoice at the ECB rate of the date the tax became chargeable, provided you apply that choice consistently. Consistency is exactly what breaks when your invoicing tool, your accounting package and your consolidation each fetch their own rate. One stored table per day and per pair gives all three the same number, with the publication date on the row, so the answer to which rate applied on which date comes from your own data instead of a screenshot of a website.