Android DateTimeFormat reports h11/h24 but formats midnight as h12/h23

[mozzius]

Environment

Android 15 / API 35 emulator, React Native 0.86.3, Hermes 250829098.0.17. Compared in the same app against Android Chromium WebView 124. Reproducer: https://github.com/mozzius/hermes-datetimeformat-repro (native Hermes and WebView side by side).

Minimal repro

const midnight = new Date('2026-05-28T21:54:00Z') // 00:54 in Europe/Helsinki

for (const [locale, hourCycle] of [
  ['en-GB', 'h11'],
  ['en-US', 'h24'],
]) {
  const formatter = new Intl.DateTimeFormat(locale, {
    timeStyle: 'short',
    hourCycle,
    timeZone: 'Europe/Helsinki',
  })
  console.log(locale, formatter.format(midnight), formatter.resolvedOptions().hourCycle)
}

Request	Android Hermes	Chromium WebView
en-GB, explicit h11	12:54 am, reports h11	00:54 am, reports h11
en-US, explicit h24	00:54, reports h24	24:54, reports h24

The day-period spacing varies by engine, but the hour at midnight is the issue: h11 must start at 0 and h24 at 24. Hermes returns the opposite cycle's hour despite reporting the requested cycle. This also affects hour12 selection: en-GB + hour12: true resolves to h11 while formatting midnight as 12:54 am; en-US + hour12: false resolves to h24 while formatting 00:54.

Source observations

• lib/Platform/Intl/java/com/facebook/hermes/intl/DateTimeFormat.java pairs a default h23 with h11 and h12 with h24 for hour12, rather than selecting the locale's preferred 12-/24-hour cycle.
• PlatformDateTimeFormatterICU.java maps both H11 and H12 to an h skeleton and both H23 and H24 to a k skeleton. The API 35 formatter subsequently formats 12 or 00 in the explicit midnight cases above.
• The Android Test262 harness skips DateTimeFormat/prototype/resolvedOptions/hourCycle-timeStyle.js.

This is distinct from the Apple backend's ignored styled hour12 request in #2208. Android does honor that request and formats the Bluesky case as a 12-hour time; its incorrect midnight semantics need an Android-specific fix and regression tests. I have not tested pre-API-24 Android's separate java.text implementation.