We use the Mastercard reference rate. Let’s assume it's Wednesday and you pay for a coffee and a croissant abroad with your neon card, you will be charged the exchange rate from Tuesday. In other words, the exchange rate from the day before your card payment is applied. So far, so good.
Now it gets a bit more complicated: Mastercard is an international company, so the different time zones play a role. If you order something totally necessary from a foreign online shop late at night at 11pm, the exchange rate from the same day can be applied due to the time difference. As mentioned, the reason for this is that Mastercard refers to a different time zone in its accounting. In short, depending on the time of day, you will be charged the exchange rate of the same day or the previous day.