From e1cbb705273a261d26b489f35a7ddca6567d9663 Mon Sep 17 00:00:00 2001
From: Igor Makowski <56691628+Mnigos@users.noreply.github.com>
Date: Tue, 22 Sep 2026 00:34:12 +0200
Subject: [PATCH] fix(web): the custom snooze calendar starts the week where
 the locale does (#12745)

---
 .../web/src/components/CustomSnoozeDialog.tsx |  2 ++
 apps/web/src/timestampFormat.test.ts          | 26 +++++++++++++++++
 apps/web/src/timestampFormat.ts               | 29 +++++++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/apps/web/src/components/CustomSnoozeDialog.tsx b/apps/web/src/components/CustomSnoozeDialog.tsx
index d80ebf0d36fb9ed1cba2227d87ab47958a4544b4..453efb1cbe5367d132a8fa006158843e4805ecb8 100644
--- a/apps/web/src/components/CustomSnoozeDialog.tsx
+++ b/apps/web/src/components/CustomSnoozeDialog.tsx
@@ -10,6 +10,7 @@ import {
 import { Button } from "./ui/button";
 import { CalendarIcon } from "lucide-react";
 import { Calendar } from "./ui/calendar";
+import { weekStartsOn } from "../timestampFormat";
 import { Popover, PopoverTrigger, PopoverPopup } from "./ui/popover";
 import { Input } from "./ui/input";
 import { Label } from "./ui/label";
@@ -149,6 +150,7 @@ function CustomSnoozeDialog() {
                             required
                             selected={date}
                             defaultMonth={date}
+                            {...(weekStartsOn === undefined ? {} : { weekStartsOn })}
                             disabled={{ before: new Date(new Date().setHours(0, 0, 0, 0)) }}
                             onSelect={(selected) => {
                               setDate(selected);
diff --git a/apps/web/src/timestampFormat.test.ts b/apps/web/src/timestampFormat.test.ts
index 8c6287010d8f1929618f3b77cc4dcd1e25259f4e..d748853000c5c36e129f4ccca1bfc670d9a70d01 100644
--- a/apps/web/src/timestampFormat.test.ts
+++ b/apps/web/src/timestampFormat.test.ts
@@ -52,6 +52,32 @@ describe("formatShortTimestamp", () => {
   });
 });
 
+describe("resolveWeekStartsOn", () => {
+  it.each([
+    ["en-US", 0],
+    ["en-GB", 1],
+    ["pl-PL", 1],
+    ["ar-EG", 6],
+  ])("starts the %s week on weekday %i", async (locale, weekday) => {
+    const { resolveWeekStartsOn } = await import("./timestampFormat");
+    expect(resolveWeekStartsOn(locale)).toBe(weekday);
+  });
+
+  it("leaves the default to the caller for a malformed locale", async () => {
+    const { resolveWeekStartsOn } = await import("./timestampFormat");
+    expect(resolveWeekStartsOn("not a locale")).toBeUndefined();
+  });
+
+  it("follows the locale the desktop host reports", async () => {
+    vi.stubGlobal("window", { desktopBridge: { getSystemLocale: () => "en-GB" } });
+    vi.resetModules();
+    const { weekStartsOn } = await import("./timestampFormat");
+    expect(weekStartsOn).toBe(1);
+    vi.unstubAllGlobals();
+    vi.resetModules();
+  });
+});
+
 describe("formatChatTimestampTooltip", () => {
   afterEach(() => {
     vi.restoreAllMocks();
diff --git a/apps/web/src/timestampFormat.ts b/apps/web/src/timestampFormat.ts
index 9dd463bb50fa396c3511d6d99e476c158c1bb452..5106a00bdf2f4938d24c01760313348ab0554d34 100644
--- a/apps/web/src/timestampFormat.ts
+++ b/apps/web/src/timestampFormat.ts
@@ -53,6 +53,35 @@ function readHostSystemLocale(): string | null {
 
 const timestampLocale = resolveTimestampLocale(readHostSystemLocale());
 
+const WEEKDAY_INDEXES = [0, 1, 2, 3, 4, 5, 6] as const;
+type WeekdayIndex = (typeof WEEKDAY_INDEXES)[number];
+
+type LocaleWithWeekInfo = Intl.Locale & {
+  readonly weekInfo?: { readonly firstDay: number };
+  getWeekInfo?: () => { readonly firstDay: number };
+};
+
+/**
+ * First weekday of a locale as a `Date#getDay` index (0 is Sunday), or
+ * `undefined` when the runtime has no week data, so callers keep their own
+ * default. Without a locale it reads the runtime's.
+ */
+export function resolveWeekStartsOn(locale: string | undefined): WeekdayIndex | undefined {
+  try {
+    const resolved: LocaleWithWeekInfo = new Intl.Locale(
+      locale ?? Intl.DateTimeFormat().resolvedOptions().locale,
+    );
+    // Week info counts Monday as 1 and Sunday as 7.
+    const firstDay = resolved.getWeekInfo?.().firstDay ?? resolved.weekInfo?.firstDay;
+    return firstDay === undefined ? undefined : WEEKDAY_INDEXES[firstDay % 7];
+  } catch {
+    return undefined;
+  }
+}
+
+/** Week start for calendars, from the same locale timestamps are shown in. */
+export const weekStartsOn = resolveWeekStartsOn(timestampLocale);
+
 const timestampFormatterCache = new Map<string, Intl.DateTimeFormat>();
 
 function getTimestampFormatter(
-- 
2.54.0