diff --git a/DECISIONS.md b/DECISIONS.md
index 1e8a6e6..cd7589f 100644
--- a/DECISIONS.md
+++ b/DECISIONS.md
@@ -375,3 +375,10 @@ Implementation decisions not spelled out in `memory-bank/implementation-plan.md`
 - **Requests:** `:params` are filled from the input; remaining fields go in the query string for GET and the JSON body for POST. `credentials: "include"` sends the session cookie. The base URL defaults to same origin (decision 0.5.20).
 - **Errors:** a response failing its schema throws `ApiValidationError`; a non-2xx response throws `ApiError` with the server's code and message (or `internal-error` if the body is not the documented format).
 - **ESLint now ignores unused names starting with `_`**, for deliberately unused parameters.
+
+## Step 8.3
+
+- **The clock offset lives in a Zustand store** (`useClock`), as the tech stack prescribes for real-time client state. `serverNow()` is `Date.now() + offset`; the device clock only affects visuals and is never trusted for outcomes.
+- **Sync uses the midpoint of the round trip:** offset = server time − (sent + received) / 2.
+- **A one-second ticker** (`startClockTicker`) refreshes `now` in the store so components re-render; components read `useClock(s => s.now)`.
+- **The header shows in-game time** as `YYYY-MM-DD HH:MM` (UTC), from sim `realToGame`.
diff --git a/apps/web/src/clock/GameClock.tsx b/apps/web/src/clock/GameClock.tsx
new file mode 100644
index 0000000..4624e4b
--- /dev/null
+++ b/apps/web/src/clock/GameClock.tsx
@@ -0,0 +1,14 @@
+import { content } from "@deadhead/content";
+import { realToGame } from "@deadhead/sim";
+import { useClock } from "./clock";
+
+/** "2100-03-14 08:00": the in-game date and hour, in UTC. */
+export function formatGameTime(gameTime: number): string {
+  return new Date(gameTime).toISOString().slice(0, 16).replace("T", " ");
+}
+
+/** The ticking in-game clock shown in the header. */
+export function GameClock() {
+  const now = useClock((state) => state.now);
+  return <time>{formatGameTime(realToGame(now, content.config))}</time>;
+}
diff --git a/apps/web/src/clock/clock.ts b/apps/web/src/clock/clock.ts
new file mode 100644
index 0000000..c9136a4
--- /dev/null
+++ b/apps/web/src/clock/clock.ts
@@ -0,0 +1,42 @@
+import type { RealTime } from "@deadhead/sim";
+import { create } from "zustand";
+import type { ApiClient } from "../api/client";
+
+interface ClockState {
+  /** Server time minus device time. A wrong device clock only shifts visuals. */
+  offsetMs: number;
+  synced: boolean;
+  /** Server-adjusted time, refreshed every second by the ticker. */
+  now: RealTime;
+}
+
+export const useClock = create<ClockState>(() => ({
+  offsetMs: 0,
+  synced: false,
+  now: Date.now(),
+}));
+
+/** The current time on the server's clock: device time plus the synced offset. */
+export function serverNow(): RealTime {
+  return Date.now() + useClock.getState().offsetMs;
+}
+
+/**
+ * Fetches server time and stores the offset. The server's reading is assumed
+ * to be taken halfway through the round trip.
+ */
+export async function syncClock(api: ApiClient): Promise<void> {
+  const sentAt = Date.now();
+  const { realTime } = await api.call("getServerTime", {});
+  const receivedAt = Date.now();
+  const offsetMs = realTime - (sentAt + receivedAt) / 2;
+  useClock.setState({ offsetMs, synced: true, now: Date.now() + offsetMs });
+}
+
+/** Updates `now` every second. Returns a function that stops the ticker. */
+export function startClockTicker(intervalMs = 1000): () => void {
+  const tick = () => useClock.setState({ now: serverNow() });
+  tick();
+  const id = setInterval(tick, intervalMs);
+  return () => clearInterval(id);
+}
diff --git a/apps/web/src/layout/Layout.tsx b/apps/web/src/layout/Layout.tsx
index 7e50cf0..7a82582 100644
--- a/apps/web/src/layout/Layout.tsx
+++ b/apps/web/src/layout/Layout.tsx
@@ -1,15 +1,16 @@
 import type { ReactNode } from "react";
 import { NavLink, Outlet } from "react-router";
+import { GameClock } from "../clock/GameClock";
 import { SCREENS } from "../routes";
 
 export interface LayoutProps {
-  /** Shown at the top right; live values arrive with the player state and clock steps. */
+  /** Shown at the top right. Credits arrive with the player state (Step 8.5). */
   credits?: ReactNode;
   clock?: ReactNode;
 }
 
 /** Header with credits and game clock, the current screen, and a bottom tab bar. */
-export function Layout({ credits = "—", clock = "—" }: LayoutProps) {
+export function Layout({ credits = "—", clock = <GameClock /> }: LayoutProps) {
   return (
     <div className="flex min-h-dvh flex-col bg-slate-950 text-slate-100">
       <header className="sticky top-0 z-10 flex items-center justify-between gap-2 border-b border-slate-800 bg-slate-950/95 px-4 py-2">
diff --git a/apps/web/test/clock.test.tsx b/apps/web/test/clock.test.tsx
new file mode 100644
index 0000000..7aa7f4d
--- /dev/null
+++ b/apps/web/test/clock.test.tsx
@@ -0,0 +1,63 @@
+import { content } from "@deadhead/content";
+import { realToGame } from "@deadhead/sim";
+import { act, render, screen } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import type { ApiClient } from "../src/api/client";
+import { serverNow, startClockTicker, syncClock, useClock } from "../src/clock/clock";
+import { formatGameTime, GameClock } from "../src/clock/GameClock";
+
+const DEVICE_NOW = Date.parse("2100-01-01T00:00:00Z");
+const TEN_MINUTES = 10 * 60_000;
+
+/** An API whose server clock runs `aheadMs` ahead of the device. */
+function serverAhead(aheadMs: number): ApiClient {
+  return {
+    call: vi.fn(async () => ({ realTime: Date.now() + aheadMs, gameTime: 0 })),
+  } as unknown as ApiClient;
+}
+
+describe("server-synced clock", () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    vi.setSystemTime(DEVICE_NOW);
+    useClock.setState({ offsetMs: 0, synced: false, now: DEVICE_NOW });
+  });
+  afterEach(() => vi.useRealTimers());
+
+  it("returns a time 10 minutes ahead when the server is 10 minutes ahead", async () => {
+    await syncClock(serverAhead(TEN_MINUTES));
+    expect(useClock.getState().synced).toBe(true);
+    expect(serverNow()).toBe(DEVICE_NOW + TEN_MINUTES);
+
+    vi.advanceTimersByTime(5000);
+    expect(serverNow()).toBe(DEVICE_NOW + TEN_MINUTES + 5000);
+  });
+
+  it("works when the server is behind the device", async () => {
+    await syncClock(serverAhead(-30_000));
+    expect(serverNow()).toBe(DEVICE_NOW - 30_000);
+  });
+
+  it("ticks the store every second", async () => {
+    await syncClock(serverAhead(TEN_MINUTES));
+    const stop = startClockTicker();
+    vi.advanceTimersByTime(3000);
+    expect(useClock.getState().now).toBe(DEVICE_NOW + TEN_MINUTES + 3000);
+    stop();
+    vi.advanceTimersByTime(3000);
+    expect(useClock.getState().now).toBe(DEVICE_NOW + TEN_MINUTES + 3000);
+  });
+
+  it("shows the in-game time in the game clock and keeps it ticking", async () => {
+    await syncClock(serverAhead(0));
+    const stop = startClockTicker();
+    render(<GameClock />);
+    const at = (real: number) => formatGameTime(realToGame(real, content.config));
+    expect(screen.getByText(at(DEVICE_NOW))).toBeInTheDocument();
+
+    // One real minute is one game hour.
+    act(() => vi.advanceTimersByTime(60_000));
+    expect(screen.getByText(at(DEVICE_NOW + 60_000))).toBeInTheDocument();
+    stop();
+  });
+});
diff --git a/memory-bank/progress.md b/memory-bank/progress.md
index 612745c..ef7fa84 100644
--- a/memory-bank/progress.md
+++ b/memory-bank/progress.md
@@ -79,6 +79,6 @@
 
 - [x] **Step 8.1 — Vite + React app.** `apps/web`: Vite 8, React 19, Tailwind 4, React Router 8. Layout (header with credits and game clock, main area, fixed six-tab bottom navigation) and placeholder screens for HQ, Contracts, Fleet, Pilots, Market, Locations. Component tests in jsdom. Dev server proxies `/api` to `127.0.0.1:3000`. Manual 375px check left to the user.
 - [x] **Step 8.2 — Typed API client.** `apps/web/src/api/client.ts`: `createApiClient().call(name, input)` built from the route catalog: path params filled, GET query / POST JSON body, `credentials: "include"`, responses validated (`ApiValidationError`), server errors raised as `ApiError` with code and message.
-- [ ] Step 8.3 — Server-synced clock
+- [x] **Step 8.3 — Server-synced clock.** `apps/web/src/clock`: Zustand `useClock` store, `syncClock(api)` (offset from server time, half round trip), `serverNow()`, `startClockTicker()` (1 s), and a `GameClock` header component showing in-game time.
 - [ ] Step 8.4 — Guest onboarding
 - [ ] Step 8.5 — Player state store