From 6381f0221f5543fa7afd90706360b7c97e599da1 Mon Sep 17 00:00:00 2001
From: neon <18559658+neonsolstice@users.noreply.github.com>
Date: Sat, 26 Sep 2026 13:41:33 +0000
Subject: [PATCH] fix(library): honor browser clock for cron schedules

Use the browser's hour cycle when its language matches the selected interface language, preserving regional time preferences.

Closes #1559
---
 .../library/utils/__tests__/cron.spec.ts      | 39 ++++++++++++++++++-
 client/src/features/library/utils/cron.ts     |  6 ++-
 client/src/stores/__tests__/locale.spec.ts    |  1 +
 3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/client/src/features/library/utils/__tests__/cron.spec.ts b/client/src/features/library/utils/__tests__/cron.spec.ts
index c82d86357..3b7e60553 100644
--- a/client/src/features/library/utils/__tests__/cron.spec.ts
+++ b/client/src/features/library/utils/__tests__/cron.spec.ts
@@ -1,8 +1,21 @@
-import { describe, expect, it } from 'vitest'
+import { afterEach, describe, expect, it, vi } from 'vitest'
 import { isFiveFieldCronExpression } from '@bookorbit/types'
 
 import { parseCronToHuman } from '../cron'
 
+function mockBrowserClock(locale: string, hour12: boolean): void {
+  const NativeDateTimeFormat = Intl.DateTimeFormat
+  vi.spyOn(Intl, 'DateTimeFormat').mockImplementation(function dateTimeFormat(requestedLocale, options) {
+    const formatter = new NativeDateTimeFormat(requestedLocale, options)
+    if (requestedLocale === undefined) {
+      vi.spyOn(formatter, 'resolvedOptions').mockReturnValue({ ...formatter.resolvedOptions(), locale, hour12 })
+    }
+    return formatter
+  })
+}
+
+afterEach(() => vi.restoreAllMocks())
+
 describe('isFiveFieldCronExpression', () => {
   it.each(['0 4 * * *', '*/30 * * * *', '0 0 * * 1', '0 4 * * MON', '0  4 * * *', '1-5/2 * * * *', '0 4 * jan-mar *'])(
     'accepts %s and previews it',
@@ -21,6 +34,30 @@ describe('isFiveFieldCronExpression', () => {
 })
 
 describe('parseCronToHuman', () => {
+  it('uses the browser clock for an English regional preference', () => {
+    mockBrowserClock('en-GB', false)
+
+    expect(parseCronToHuman('31 12,0 * * *', 'en')).toBe('At 00:31 and 12:31')
+  })
+
+  it('preserves a 12-hour English browser preference', () => {
+    mockBrowserClock('en-US', true)
+
+    expect(parseCronToHuman('31 12,0 * * *', 'en')).toBe('At 12:31 AM and 12:31 PM')
+  })
+
+  it('uses the selected language convention when it differs from the browser language', () => {
+    mockBrowserClock('en-US', true)
+
+    expect(parseCronToHuman('31 12,0 * * *', 'nl')).toBe('At 00:31 and 12:31')
+  })
+
+  it('preserves the English convention with a Dutch browser language', () => {
+    mockBrowserClock('nl-BE', false)
+
+    expect(parseCronToHuman('31 12,0 * * *', 'en')).toBe('At 12:31 AM and 12:31 PM')
+  })
+
   it('returns null for null input', () => {
     expect(parseCronToHuman(null, 'en')).toBeNull()
   })
diff --git a/client/src/features/library/utils/cron.ts b/client/src/features/library/utils/cron.ts
index 82f3b6f75..b0abc5bf1 100644
--- a/client/src/features/library/utils/cron.ts
+++ b/client/src/features/library/utils/cron.ts
@@ -3,7 +3,11 @@ import cronstrue from 'cronstrue'
 export function parseCronToHuman(cron: string | null | undefined, locale: string): string | null {
   if (!cron) return null
   try {
-    const use24HourTimeFormat = new Intl.DateTimeFormat(locale, { hour: 'numeric' }).resolvedOptions().hour12 === false
+    const browserTimeFormat = new Intl.DateTimeFormat(undefined, { hour: 'numeric' }).resolvedOptions()
+    const localeTimeFormat = new Intl.DateTimeFormat(locale, { hour: 'numeric' }).resolvedOptions()
+    const browserLanguage = new Intl.Locale(browserTimeFormat.locale).language
+    const localeLanguage = new Intl.Locale(localeTimeFormat.locale).language
+    const use24HourTimeFormat = (browserLanguage === localeLanguage ? browserTimeFormat : localeTimeFormat).hour12 === false
     return cronstrue.toString(cron, { use24HourTimeFormat, verbose: false })
   } catch {
     return cron
diff --git a/client/src/stores/__tests__/locale.spec.ts b/client/src/stores/__tests__/locale.spec.ts
index 73d44d37f..41bb1259a 100644
--- a/client/src/stores/__tests__/locale.spec.ts
+++ b/client/src/stores/__tests__/locale.spec.ts
@@ -29,6 +29,7 @@ describe('locale store', () => {
   it('matches exact locales before falling back to a supported base language', async () => {
     const { matchSupportedLocale } = await import('../locale')
 
+    expect(matchSupportedLocale(['en', 'en-GB', 'en-US'])).toBe('en')
     expect(matchSupportedLocale(['nl-NL', 'en-US'])).toBe('nl')
     expect(matchSupportedLocale(['de-DE', 'en-GB'])).toBe('de')
     expect(matchSupportedLocale(['el-GR', 'en-GB'])).toBe('el')