They should be converted according to the terms of your agreement, which probably (vaguely) describes how the rates are set. The rates are established by the transaction network, not by the card issuer. Credit card transactions are typically converted based on the posting date, not the transaction date.

There is no "official" exchange rate, and rates do change continuously, but I would expect ATM rates to remain the same for an entire day, based on "a" rate from the previous day.