Arithmetic is not eligibility
86Hours can calculate a result deterministically from the chosen inputs; it does not decide that a customer is legally entitled to claim it.
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Late-payment interest
Calculate from transparent inputs, then check eligibility.
A calculation needs the eligible principal, relevant dates and annual rate. Whether statutory or contractual interest applies is a separate question that depends on the transaction and current rules.
Keep the working
Record the rate, day count, calculation date, rounding and source used so the figure can be reviewed.