The agreed conditions for payment, including the due-date trigger, time allowed and other relevant requirements. The number of days only makes sense alongside the event that starts the clock and any conditions for a valid invoice.
An example
If an agreement says Net 30 from invoice date, an invoice dated 1 September is due on 1 October using ordinary calendar-day counting, subject to the agreement’s rules. A receipt-based trigger could give a different date.
What to check
Do not assume that a day count identifies the trigger. Confirm the actual wording, applicable parties and any amendments.
In practice
Read the actual clause and any order-specific changes. Keep the payment period separate from how often invoices are issued, and check whether a purchase order or acceptance step affects the agreed timing.