It is quite common to invoice a customer with terms of “30 days or nett monthly account”. Small customers will be expected to pay within 30 days, while large customers will be expected to pay at the end of the month following the month of the invoice, so an invoice sent in February 2019 would be settled by Sunday 31 March 2019.
Large companies do it this way because they may receive several invoices from a supplier, and will want to settle all of them with a single payment when they do their computerised cheque run. It would therefore be a good idea for the supplier to have sent a statement at the start of March listing all outstanding invoices. Typically the cheque run would be about the 25th of the month, which is a Monday this month. If you give credit and have debts to collect, then you might like to have a discussion with us. Most accountants are also general business advisers as well.
The “nett” in “nett monthly account” has “net” as an alternative spelling.