Night Runner states something that is became obvious to me after two years as a small, one-person, business owner.
Take some advise. Use a 30-day net payment for invoices, and assume you'll paid in 45 days. If your invoice is sent out a little late --after the other companies billing cutoff date-- add 30 more days to expect payment.
Big companies don't process and pay invoices as they receive them. Their accounting departments process different paperwork (accounts receivable, accounts payable, etc.) at different time during the month. That's way an invoice that is late 1 day, based on their internal calendar, might not get processed for an additional 30 days.