In a signed prommissory note, what exactly does the following mean…

“unpaid principal after the due date shall accrue interest at an annual rate of 15.00 percent until paid.”

Would an example on $100 that is 30 days late be…

$100 x 15% = $15.00, divided by 12 months = $1.25 charged per month? Did I figure that right, or does it mean $15 each month?