diff --git a/src/client/GameModeSelector.ts b/src/client/GameModeSelector.ts
index 6cf39a873..0163e8c68 100644
--- a/src/client/GameModeSelector.ts
+++ b/src/client/GameModeSelector.ts
@@ -18,8 +18,10 @@ import { JoinLobbyEvent } from "./Main";
 import { SinglePlayerModal } from "./SinglePlayerModal";
 import { terrainMapFileLoader } from "./TerrainMapFileLoader";
 import {
+  calculateServerTimeOffset,
   getMapName,
   getModifierLabels,
+  getSecondsUntilServerTimestamp,
   renderDuration,
   translateText,
 } from "./Utils";
@@ -81,7 +83,7 @@ export class GameModeSelector extends LitElement {
 
   private handleLobbiesUpdate(lobbies: PublicGames) {
     this.lobbies = lobbies;
-    this.serverTimeOffset = lobbies.serverTime - Date.now();
+    this.serverTimeOffset = calculateServerTimeOffset(lobbies.serverTime);
     document.dispatchEvent(
       new CustomEvent("public-lobbies-update", {
         detail: { payload: lobbies },
@@ -279,12 +281,7 @@ export class GameModeSelector extends LitElement {
     const useContain =
       aspectRatio !== undefined && (aspectRatio > 4 || aspectRatio < 0.25);
     const timeRemaining = lobby.startsAt
-      ? Math.max(
-          0,
-          Math.floor(
-            (lobby.startsAt - this.serverTimeOffset - Date.now()) / 1000,
-          ),
-        )
+      ? getSecondsUntilServerTimestamp(lobby.startsAt, this.serverTimeOffset)
       : undefined;
 
     let timeDisplay: string = "";
diff --git a/src/client/JoinLobbyModal.ts b/src/client/JoinLobbyModal.ts
index a05d768c9..4bea53d43 100644
--- a/src/client/JoinLobbyModal.ts
+++ b/src/client/JoinLobbyModal.ts
@@ -1,9 +1,12 @@
 import { html, TemplateResult } from "lit";
 import { customElement, property, query, state } from "lit/decorators.js";
 import {
+  calculateServerTimeOffset,
   getActiveModifiers,
   getGameModeLabel,
   getMapName,
+  getSecondsUntilServerTimestamp,
+  getServerNow,
   renderDuration,
   renderNumber,
   translateText,
@@ -44,6 +47,7 @@ export class JoinLobbyModal extends BaseModal {
   @state() private currentClientID: string = "";
   @state() private nationCount: number = 0;
   @state() private lobbyStartAt: number | null = null;
+  @state() private serverTimeOffset: number = 0;
   @state() private isConnecting: boolean = true;
   @state() private lobbyCreatorClientID: string | null = null;
 
@@ -77,7 +81,10 @@ export class JoinLobbyModal extends BaseModal {
     // Post-join state: show lobby info (identical for public & private)
     const secondsRemaining =
       this.lobbyStartAt !== null
-        ? Math.max(0, Math.floor((this.lobbyStartAt - Date.now()) / 1000))
+        ? getSecondsUntilServerTimestamp(
+            this.lobbyStartAt,
+            this.serverTimeOffset,
+          )
         : null;
     const statusLabel =
       secondsRemaining === null
@@ -328,6 +335,7 @@ export class JoinLobbyModal extends BaseModal {
     this.players = [];
     this.nationCount = 0;
     this.lobbyStartAt = null;
+    this.serverTimeOffset = 0;
     this.lobbyCreatorClientID = null;
     this.isConnecting = true;
     this.handledJoinTimeout = false;
@@ -377,6 +385,7 @@ export class JoinLobbyModal extends BaseModal {
     this.currentClientID = "";
     this.nationCount = 0;
     this.lobbyStartAt = null;
+    this.serverTimeOffset = 0;
     this.lobbyCreatorClientID = null;
     this.isConnecting = true;
     this.leaveLobbyOnClose = true;
@@ -513,6 +522,9 @@ export class JoinLobbyModal extends BaseModal {
 
   private updateFromLobby(lobby: GameInfo | PublicGameInfo) {
     this.players = "clients" in lobby ? (lobby.clients ?? []) : [];
+    if ("serverTime" in lobby && typeof lobby.serverTime === "number") {
+      this.serverTimeOffset = calculateServerTimeOffset(lobby.serverTime);
+    }
     this.lobbyStartAt = lobby.startsAt ?? null;
     this.syncCountdownTimer();
     if (lobby.gameConfig) {
@@ -577,7 +589,7 @@ export class JoinLobbyModal extends BaseModal {
     ) {
       return;
     }
-    if (Date.now() < this.lobbyStartAt) {
+    if (getServerNow(this.serverTimeOffset) < this.lobbyStartAt) {
       return;
     }
     this.handledJoinTimeout = true;
diff --git a/src/client/Utils.ts b/src/client/Utils.ts
index c9323bead..a775a6ae6 100644
--- a/src/client/Utils.ts
+++ b/src/client/Utils.ts
@@ -628,3 +628,30 @@ export function getDiscordAvatarUrl(user: {
 
   return null;
 }
+export function calculateServerTimeOffset(
+  serverTimeMs: number,
+  localNowMs: number = Date.now(),
+): number {
+  return serverTimeMs - localNowMs;
+}
+
+export function getServerNow(
+  serverTimeOffsetMs: number,
+  localNowMs: number = Date.now(),
+): number {
+  return localNowMs + serverTimeOffsetMs;
+}
+
+export function getSecondsUntilServerTimestamp(
+  targetServerTimestampMs: number,
+  serverTimeOffsetMs: number,
+  localNowMs: number = Date.now(),
+): number {
+  return Math.max(
+    0,
+    Math.floor(
+      (targetServerTimestampMs - getServerNow(serverTimeOffsetMs, localNowMs)) /
+        1000,
+    ),
+  );
+}
diff --git a/tests/client/JoinLobbyModal.test.ts b/tests/client/JoinLobbyModal.test.ts
new file mode 100644
index 000000000..53e9b35de
--- /dev/null
+++ b/tests/client/JoinLobbyModal.test.ts
@@ -0,0 +1,74 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { JoinLobbyModal } from "../../src/client/JoinLobbyModal";
+
+describe("JoinLobbyModal server time offset", () => {
+  let nowMs = 0;
+
+  beforeEach(() => {
+    vi.spyOn(Date, "now").mockImplementation(() => nowMs);
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+  });
+
+  it("updates serverTimeOffset from lobby serverTime", () => {
+    const modal = new JoinLobbyModal();
+    (modal as any).syncCountdownTimer = vi.fn();
+
+    nowMs = 220_000;
+    (modal as any).updateFromLobby({
+      gameID: "g1",
+      serverTime: 200_000,
+      startsAt: 230_000,
+      clients: [],
+    });
+
+    expect((modal as any).serverTimeOffset).toBe(-20_000);
+    expect((modal as any).lobbyStartAt).toBe(230_000);
+  });
+
+  it("does not trigger join timeout early when local clock is ahead", () => {
+    const modal = new JoinLobbyModal();
+    const closeSpy = vi
+      .spyOn(modal, "closeAndLeave")
+      .mockImplementation(() => undefined);
+    const dispatchSpy = vi.spyOn(window, "dispatchEvent");
+
+    (modal as any).isModalOpen = true;
+    (modal as any).isConnecting = true;
+    (modal as any).handledJoinTimeout = false;
+
+    // Local clock is +60s ahead of server clock.
+    nowMs = 160_000;
+    (modal as any).lobbyStartAt = 105_000;
+    (modal as any).serverTimeOffset = -60_000;
+
+    (modal as any).checkForJoinTimeout();
+
+    expect(closeSpy).not.toHaveBeenCalled();
+    expect(dispatchSpy).not.toHaveBeenCalled();
+    expect((modal as any).handledJoinTimeout).toBe(false);
+  });
+
+  it("triggers join timeout once adjusted server time reaches lobbyStartAt", () => {
+    const modal = new JoinLobbyModal();
+    const closeSpy = vi
+      .spyOn(modal, "closeAndLeave")
+      .mockImplementation(() => undefined);
+    const dispatchSpy = vi.spyOn(window, "dispatchEvent");
+
+    (modal as any).isModalOpen = true;
+    (modal as any).isConnecting = true;
+    (modal as any).handledJoinTimeout = false;
+    (modal as any).lobbyStartAt = 105_000;
+    (modal as any).serverTimeOffset = -60_000;
+
+    nowMs = 165_000;
+    (modal as any).checkForJoinTimeout();
+
+    expect(closeSpy).toHaveBeenCalledTimes(1);
+    expect(dispatchSpy).toHaveBeenCalledTimes(1);
+    expect((modal as any).handledJoinTimeout).toBe(true);
+  });
+});