I just filed my tax return and paid the amount owing too, five days before the due date.
Tax office told me they assume I probably won't pay until oh, say… (wet finger in the wind) maybe May 6. They calculate an arrears interest charge for then and add it to my tab.
On phone they asked: "Oh you didn't pay that did you? It'll just go away when we settle the account."
They pre-charge a late fee before anything is late.