There are however companies that have started using Average Daily Balance for credit cards (Example: Ulster Bank)
Average Daily Balance.
It credits your account from the day payment is received by the issuer. To figure the balance due, the issuer totals the beginning balance for each day in the billing period and subtracts any credits made to your account that day. While new purchases may or may not be added to the balance, depending on your plan, cash advances typically are included. The resulting daily balances are added for the billing cycle. The total is then divided by the number of days in the billing period to get the "average daily balance."