Net 30 payment terms: calculate the due date without ambiguity

Write the starting event and the date

For this example, the parties agree that payment is due 30 calendar days after the invoice issue date, excluding the issue date itself from the count. An invoice issued September 21, 2026 is therefore due October 21, 2026. This is the counting convention for the example, not a claim about a mandatory legal deadline.

Payment is due on October 21, 2026, which is 30 calendar days after the invoice issue date of September 21, 2026. Please include invoice INV-2026-021 as the payment reference.

Compare the same invoice under different agreements

The customer receiving the invoice later does not automatically change an issue-date-based agreement. A receipt-based agreement uses a different starting event. State which one applies before billing.

• Do not substitute “one month” for 30 days without checking the agreement.
• Decide how weekends or holidays are handled rather than silently extending a deadline.
• Use the customer’s purchase-order or contract terms where applicable; resolve conflicting dates before sending.

Do not turn a quote expiry into an invoice deadline

The “Due / valid date” field serves different purposes in the quote and invoice tools. A quote valid through September 25 does not establish that the invoice must be paid by September 25. On a converted invoice, enter the actual payment deadline and review its Terms field.

• Offer validity and acceptance dates
• Write a milestone payment schedule

Check the remaining balance before a reminder

A USD 750 invoice with a confirmed USD 250 payment has USD 500 outstanding. The due date does not change that arithmetic. Update the invoice’s Payment already received field and ask for USD 500, not a second USD 750.

Do not add a late fee merely because an invoice passes its due date. Whether a fee is permitted depends on the agreement and applicable rules; this guide does not determine either.

• Use a balance-reminder email