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Locale-aware Formats

MUST

MUST: Locale-aware dates/times/numbers/currency

Format dates, times, numbers, and currency for user locale

Bad

Good

Why it matters

Different regions format data differently (12/31/2024 vs 31.12.2024, $1,000.00 vs 1.000,00€). Use Intl API in JavaScript to format dates, numbers, and currency according to the user's locale automatically.

References

Built by Gleb Stroganov, design engineer at Evil Martians.

The rules come from other people's skills and guidelines — Vercel, Rauno Freiberg, @Ibelick, impeccable, Emil Kowalski, Tailwind, RAMS — each one credited on the Sources page. The work here is extraction and wiring: every rule is pulled into one corpus, given a good and a bad example you can operate, a MUST/SHOULD/NEVER rule an agent can paste, and a link back to where it came from.