I recently started doing freelance work for a small-to-medium sized company. Per the terms of the agreement, on April 1 I submitted an invoice for all of my hours for March. We never discussed due date on invoices, but in my experience I usually get a check in the mail after 2-3 weeks at the most, and I simply put "due on receipt" for every invoice I create.
On April 26th, having not received payment yet, I sent a brief and friendly email asking about the status of the invoice and payment. I was told the company "pays at 30".
I understand the premise of "net D" billing, in the context of one business giving another business a specified period of time to pay before it's considered late. As a contractor though I don't understand why a company would sit on an invoice for 30 days before sending payment. What would be the purpose of this other than "that's our policy"?