Tilly Ashworth

The subject is small and the disagreements are large, so I state the convention on every page rather than assume one. A change from eight to ten is an increase of twenty-five percent, and ten is one hundred and twenty-five percent of eight, and both statements are true and neither is the answer to the other question. Percentage points versus percent is the same trap in another guise, and it appears in reporting often enough to be worth naming separately.

Counting is where dates go wrong. Whether you count both endpoints decides the answer to every age and duration question, and the two conventions give different results for every date range rather than occasionally. Same-day birthdays are one case where the choice is visible.

Leap years are the second. Every four years except centuries not divisible by four hundred, and the rule has an exception that catches people born in or around one. Time zones are the third, and they matter more than they appear to because a date is an instant plus a place, and the place determines the calendar day. Round at the end is the fourth, and it is arithmetic rather than convention, but it is the one calculators get wrong most visibly.

I would rather a calculator state its conventions on the page than be quietly right on one interpretation and quietly wrong on the other. State the convention and the answer is arguable. Hide it and the answer is wrong somewhere without either implementation being at fault.