If function with dates

[comment]

Well, if we're going to nitpick, I wasn't mixing the two - just drawing an analogy. There's no commandment to charge late fees, and if a business decides to charge them, they can also decide to rescind them in individual cases - either fully or partially. How this goes into the books is another matter, and I believe the size of the business can play a role here. I know for sure my CPA would shrug over the differences and call them "taxable income".