fix: a calendar date is the same day everywhere; legal pages use the shared language picker

formatDate read a date-only value as UTC midnight and showed it in the viewer's zone, so the legal pages' last update read a day early west of UTC. Date-only values are shown in UTC. The legal pages' hand-rolled language buttons are the landing page's LanguageSwitcher. formatDateTime and formatDate get the two-language test formatNumber already had.

frontend/src/lib/format.test.ts

@@ -1,6 +1,8 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import {
   RELATIVE_UNITS,
+  formatDate,
+  formatDateTime,
   formatNumber,
   formatRuntime,
   formatTotalHours,
@@ -191,3 +193,33 @@ describe("formatRuntime", () => {
     }
   });
 });
+
+// Dates had the same bug as numbers (a bare `toLocaleString()`), and the same trap for a test: the
+// runtime's own locale and time zone are the dev box's, so both languages — and a zone west of UTC —
+// are pinned explicitly.
+describe("formatDateTime / formatDate", () => {
+  const savedTz = process.env.TZ;
+  afterEach(() => {
+    i18nStub.language = "en";
+    if (savedTz === undefined) delete process.env.TZ;
+    else process.env.TZ = savedTz;
+  });
+
+  it("formats a timestamp in the UI language, not in the runtime's", () => {
+    const iso = "2026-09-17T08:30:14Z";
+    const asEnglish = formatDateTime(iso);
+    i18nStub.language = "hu";
+    const asHungarian = formatDateTime(iso);
+    expect(asEnglish).toBe(new Date(iso).toLocaleString("en"));
+    expect(asHungarian).toBe(new Date(iso).toLocaleString("hu"));
+    expect(asEnglish).not.toBe(asHungarian);
+    expect(formatDate(iso, "en")).not.toBe(formatDate(iso, "hu"));
+  });
+
+  it("shows a date-only value as that calendar day in every time zone", () => {
+    process.env.TZ = "America/New_York";
+    expect(formatDate("2026-09-17", "en")).toBe("Sep 17, 2026");
+    process.env.TZ = "Pacific/Kiritimati";
+    expect(formatDate("2026-09-17", "en")).toBe("Sep 17, 2026");
+  });
+});

frontend/src/lib/format.ts

@@ -176,8 +176,16 @@ export function formatDateTime(iso: string | null | undefined): string {
 }
 
 // Absolute day-precision date (e.g. "12 Jan 2017"), shown alongside a relative time. Empty for null.
+// A DATE-only value ("2026-09-17": a release, the legal pages' last update) is a calendar day, not an
+// instant: `new Date` reads it as UTC midnight, which is still the day before west of UTC, so it is
+// shown in UTC. A timestamp is shown in the viewer's own zone, as it should be.
 export function formatDate(iso: string | null | undefined, lang: string): string {
-  return iso
-    ? new Date(iso).toLocaleDateString(lang, { year: "numeric", month: "short", day: "numeric" })
-    : "";
+  if (!iso) return "";
+  const dateOnly = /^\d{4}-\d{2}-\d{2}$/.test(iso);
+  return new Date(iso).toLocaleDateString(lang, {
+    year: "numeric",
+    month: "short",
+    day: "numeric",
+    ...(dateOnly ? { timeZone: "UTC" } : {}),
+  });
 }