fix(common): handle locales with existing Unicode extension (e.g. ar-u-nu-arab)

[crossi-dev]

Fixes #68

Problem

format() throws RangeError: Invalid language tag when the locale already carries a Unicode extension block, e.g. ar-u-nu-arab (Arabic with Arabic-Indic numerals).

The cause is in createPartMap (src/common.ts): it pins the hour cycle by string-concatenating the extension onto the locale —

const preciseLocale = `${locale}-u-hc-${hour12 ? "h12" : "h23"}`

For ar-u-nu-arab this produces ar-u-nu-arab-u-hc-h23, which is invalid BCP 47 — a tag may only contain one -u- block. Intl.DateTimeFormat then throws.

Fix

Use Intl.Locale, which understands the BCP 47 grammar and merges extension subtags into a single, valid -u- block (preserving -nu-arab):

const preciseLocale = new Intl.Locale(locale, {
  hourCycle: hour12 ? "h12" : "h23",
}).toString()

One-line logic change, plus a regression test.

Verified

• New test (does not throw when locale includes a numbering system) fails before the change with the exact RangeError, passes after.
• Full suite: zero regressions — the same pre-existing failures on main (machine-local timezone-offset expectations), +1 net passing.

---

Came across Tempo and noticed #68 had been open a while, so I went ahead and fixed it. No strings attached — I'm with Choreless and we like contributing real fixes to projects we use. If it's useful and you ever want a hand with something larger, happy to help.

— Charles

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