Unless otherwise agreed, interest shall be allowed at the rate of twelve percent per annum on money due on any instrument in writing, or on settlement of the account from the day the balance shall be agreed upon, on money received to the use of another and retained without the owner’s consent, express or implied, from the receipt thereof, and on money loaned or due and withheld by unreasonable delay of payment. Unless otherwise agreed or provided by law, each charge with respect to unsettled accounts between parties shall bear interest from the date of billing unless paid within thirty days from the date of billing.