For the purposes of determining a payment due date and the date on which interest will begin to accrue if a payment is late, an invoice shall be deemed to be received: For invoices that are mailed, when a proper invoice is received by the agency, as of the date the agency annotates the invoice with the date and time of receipt; or For invoices electronically transmitted, on the date the transmission is received by the agency, or the next business day if received after 5 p.m. Eastern Time.