Does “NET 30 DAYS” on a legal invoice refer to 30 days after the billing period end date, the invoice creation date or the invoice receipt date?

Question

Does "NET 30 DAYS" on a legal invoice from a US law firm refer to:

1. 30 days after the billing period end date (the last day for which hours are billed)
2. 30 days after the invoice creation date (the date listed on the letter head of the invoice below the name and address of the legal firm)
3. or 30 days after the invoice receipt date (when I actually receive the bill)

The second one is usually 2 weeks after billing period end date and the third one 3 weeks after billing period end date.

Answer 1

It can't be (1) as the last date of service doesn't mean the bill was sent. What if they take a month to bill you?

(2) seems right, 30 days gives enough time, say 5 days, for receipt of mail, and then 3 weeks for you to pay.

(3) not so much, as it adds the gray area of transit thus losing a real 'due by'.

Answer 2

Which of the three dates are on the invoice? I have to assume only billing period (1) & the invoice creation date (2).

My first thought was the invoice creation date (2) is probably in the same area that describes the payment terms, making it therefore the correct number.

However the billing period (2) is the correct answer; Wikipedia's article on Net D reads

... trade credit which specify that the net amount (the total outstanding on the invoice) is expected to be paid in full and received by the seller within 10, 15, 30 or 60 days after the goods are dispatched or service is completed. ...

The 30 days allow for the service to be completed, an invoice created, transmitted to you, you write a check and finally they process the payment.

Of course the truest and smartest answer is to call and check.