Convention!
The most boring, but the most read, document I ever wrote was the Interest Rate Instruments and Market Conventions Guide . The document is more than 10 years old and requires a serious update. One thing that does not require an update is the convention for money market deposit in EUR. The convention is as always ACT/360 simple interest. The official ECB rates are at 4.00%, which means that if you invest today for one year a notional of 100,000 EUR, you get an interest from 15 November 2023 (T+2) to 15 November 2024 equal to 4,066.67 EUR (2024 is a leap year). If a bank wants to trick a retail customer, he can claim that you can get an interest of 4.0556% without telling you that he is using an ACT/365F convention. The amount is the same (up to the rounding) but it sounds a lot better. You will naturally tell me that this is too much of a misleading trick and that consumer protection agencies will see that immediately and remind the bank about transparency and fair advertisement. A di