fix: incorrect 24h time detection for non-AM/PM locales

packages/lib/timeFormat.test.ts

@@ -0,0 +1,40 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+vi.mock("@calcom/lib/webstorage", () => ({
+  localStorage: {
+    getItem: vi.fn().mockReturnValue(null),
+    setItem: vi.fn(),
+  },
+}));
+
+import { isBrowserLocale24h } from "./timeFormat";
+
+describe("isBrowserLocale24h", () => {
+  beforeEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("returns true for 24h hourCycle", () => {
+    vi.spyOn(Intl, "DateTimeFormat").mockImplementation(
+      function (): Intl.DateTimeFormat {
+        return {
+          resolvedOptions: () => ({ hourCycle: "h23" }),
+        } as unknown as Intl.DateTimeFormat;
+      }
+    );
+
+    expect(isBrowserLocale24h()).toBe(true);
+  });
+
+  it("returns false for 12h hourCycle", () => {
+    vi.spyOn(Intl, "DateTimeFormat").mockImplementation(
+      function (): Intl.DateTimeFormat {
+        return {
+          resolvedOptions: () => ({ hourCycle: "h12" }),
+        } as unknown as Intl.DateTimeFormat;
+      }
+    );
+
+    expect(isBrowserLocale24h()).toBe(false);
+  });
+});

packages/lib/timeFormat.ts

@@ -41,14 +41,14 @@ export const isBrowserLocale24h = () => {
   } else if (localStorageTimeFormat === false) {
     return false;
   }
-  // Intl.DateTimeFormat with value=undefined uses local browser settings.
-  if (!!new Intl.DateTimeFormat(undefined, { hour: "numeric" }).format(0).match(/M/i)) {
-    setIs24hClockInLocalStorage(false);
-    return false;
-  } else {
-    setIs24hClockInLocalStorage(true);
-    return true;
-  }
+
+  const formatter = new Intl.DateTimeFormat(undefined, { hour: "numeric" });
+  const hourCycle = (formatter.resolvedOptions() as { hourCycle?: string }).hourCycle;
+
+  const is24h = hourCycle === "h23" || hourCycle === "h24";
+
+  setIs24hClockInLocalStorage(is24h);
+  return is24h;
 };
 
 /**