Date.formatted(date:time:) ignores app locale on Android (uses JVM default)

[sebsto]

Summary

Date.formatted(date: .omitted, time: .shortened) produces AM/PM output on an Android device configured in French (fr), where it should produce 24-hour format (e.g. "14:30" not "2:30 PM").

Root Cause

In Sources/SkipFoundation/Date.swift (line ~235), the formatted(date:time:) implementation creates a bare DateFormatter() and sets dateStyle/timeStyle, but never assigns a .locale:

public func formatted(date: Date.FormatStyle.DateStyle, time: Date.FormatStyle.TimeStyle) -> String {
    let df = DateFormatter()
    // ... sets dateStyle / timeStyle ...
    return df.string(from: self)
}

When _locale is nil, DateFormatter.createDateFormat() calls java.text.DateFormat.getTimeInstance(SHORT) without a locale argument, which falls back to java.util.Locale.getDefault().

In a SkipFuse native app, java.util.Locale.getDefault() in the Swift runtime process may not match the Android app locale (it can be stuck on en_US), so French users get 12-hour AM/PM formatting.

Expected Behavior

Date.formatted(date:time:) should respect the app locale, matching Apple's Foundation behavior where .shortened produces locale-appropriate time (24h for French, 12h for English).

Suggested Fix

Set Locale.current on the DateFormatter before formatting:

public func formatted(date: Date.FormatStyle.DateStyle, time: Date.FormatStyle.TimeStyle) -> String {
    let df = DateFormatter()
    df.locale = Locale.current  // <-- add this
    // ... rest unchanged ...
    return df.string(from: self)
}

This ensures the formatter picks up the correct locale even when the JVM default diverges.

A deeper fix might also ensure Locale.current itself is synced to the Android app locale at process startup (if it isn't already), but propagating it explicitly in formatted would fix the immediate issue.

Environment

• Skip native fuse app (mode: native)
• Device language: French (fr)
• SkipFoundation via SPM (latest)
• Observed on Android emulator and physical device; iOS is unaffected (uses Apple Foundation)

[sebsto]

Update: Locale.current is also broken in native fuse mode

After applying the suggested fix (explicitly setting formatter.locale = Locale.current), the issue persists. This confirms that Locale.current itself does not reflect the Android device locale in the native fuse runtime.

Evidence

• The app UI is in French: Bundle.module.localizedString(forKey: "Played at %@") correctly returns "Diffusé à %@" (the Android resource system works).
• Yet with formatter.locale = Locale.current explicitly set, DateFormatter with .timeStyle = .short still produces "9:18 AM" instead of "9:18" / "09:18".
• The system clock in the status bar shows 11:31 (24h) — the device IS configured for French/24h.

This means Locale.current (which maps to java.util.Locale.getDefault() in SkipFoundation's Locale.swift) returns en_US (or another 12h locale) rather than fr_FR in the Swift-for-Android process.

Root cause (two layers)

1. Date.formatted(date:time:) doesn't pass any locale to the DateFormatter it creates internally (the original report).
2. Locale.current is disconnected from the Android app locale — even when explicitly set on a DateFormatter, it still produces 12h output for a French device.

The Android resource system (used by Bundle.module.localizedString) correctly resolves French, but java.util.Locale.getDefault() in the Swift process doesn't match. These two locale sources are out of sync.

Current workaround

Using a localized dateFormat pattern string from the string catalog (which goes through the working resource system):

#if os(Android)
// "h:mm a" is the catalog key; French translation is "HH:mm"
let pattern = Bundle.module.localizedString(forKey: "h:mm a", value: nil, table: nil)
let formatter = DateFormatter()
formatter.dateFormat = pattern
return formatter.string(from: date)
#endif

This bypasses both Locale.current and .timeStyle entirely.