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Calendar math

reasoning · GPT-5.4 nano · GPT-4.1 nano · Claude Haiku 4.5 · Claude Sonnet 5

The prompt (verbatim, sent to every lane)

A meetup lands on the third Wednesday of September 2026. What is the date? Reply in YYYY-MM-DD format only.
How scoring works

Exactly one YYYY-MM-DD token in the output, equal to 2026-09-16. Two candidate dates, or none in ISO format, fail. Scoring strips leading/trailing whitespace and one wrapping pair of code fences or quotes before it judges — content over ceremony. Winner rule: correct → cheapest → fastest.

Prices as listed by OpenAI and Anthropic, July 2026. Costs come from provider usage fields × list prices — nothing is invented.