How might I go about changing the payment terms or creating a payment terms code so that the money is taken on the 15th of every month?
You need to create payment terms as 15 D in payment terms and need to attach to customer master .
So during transactions it will automatically calculate due date
According to the link 15D would be in 15 days and D15 would be be every 15th day of a month.
Does that mean either 15D with a customer master attached or D15 or would solve my problem?
Well looks like it works so verified