diff --git a/app/api/auth/session/route.ts b/app/api/auth/session/route.ts
index 6a0ce0a23..e68645a30 100644
--- a/app/api/auth/session/route.ts
+++ b/app/api/auth/session/route.ts
@@ -23,7 +23,7 @@ import { revokeWopiTokens } from '@/lib/wopi/revocation';
 import { clearPairReauthInStore } from '@/lib/auth/pair-reauth';
 import { loginClientAddress } from '@/lib/auth/client-address';
 import { LOGIN_BUDGET_RETRY_AFTER_SECONDS, reserveLoginAttempt } from '@/lib/auth/verify-budget';
-import { isPastMaxAge, maxSessionHoursFor, SESSION_TOO_OLD } from '@/lib/auth/session-age';
+import { isPastMaxAge, limitsEnforcedSince, maxSessionHoursFor, SESSION_TOO_OLD } from '@/lib/auth/session-age';
 
 function sessionCookieOptions() {
   return {
@@ -77,30 +77,25 @@ function getSlot(request: NextRequest): number {
 /**
  * The organisation's maximum session length for a remembered session
  * (lib/auth/session-age.ts). A cookie from before sign-in times were
- * recorded counts as signed in now and gets the time written into it.
- * Returns the sign-in time, or null when the session is too old: its
- * cookies are cleared then.
+ * recorded counts as signed in when the server first enforced the limits,
+ * a fixed time, so a kept copy of it runs out like any other. Returns the
+ * sign-in time, or the limit when the session is too old: its cookies are
+ * cleared then.
  */
 async function checkSessionAge(
   cookieStore: Awaited<ReturnType<typeof cookies>>,
   slot: number,
   credentials: SessionCredentials,
 ): Promise<{ signedInAt: number } | { maxHours: number }> {
-  const cookieName = sessionCookieName(slot);
-  if (credentials.signedInAt === undefined) {
-    const signedInAt = Date.now();
-    const secret = credentials.token !== undefined ? { token: credentials.token } : credentials.password;
-    cookieStore.set(cookieName, encryptSession(credentials.serverUrl, credentials.username, secret, signedInAt), sessionCookieOptions());
-    return { signedInAt };
-  }
+  const signedInAt = credentials.signedInAt ?? await limitsEnforcedSince();
   // An impersonation login is `target%master`; the policy is the target's.
   const maxHours = await maxSessionHoursFor(credentials.username.split('%', 1)[0]);
-  if (isPastMaxAge(credentials.signedInAt, maxHours)) {
-    cookieStore.delete(cookieName);
+  if (isPastMaxAge(signedInAt, maxHours)) {
+    cookieStore.delete(sessionCookieName(slot));
     clearStalwartAuthContextInStore(cookieStore, slot);
     return { maxHours };
   }
-  return { signedInAt: credentials.signedInAt };
+  return { signedInAt };
 }
 
 function sessionTooOld(maxHours: number) {
@@ -209,14 +204,14 @@ export async function POST(request: NextRequest) {
       username,
       authHeader,
       ...(accountName !== username ? { accountName } : {}),
-      signedInAt,
     });
 
     void recordLogin(username, normalizedServerUrl);
 
     const warning = insecureCookieHint(request);
     if (warning) logger.warn(`session: ${warning}`);
-    return NextResponse.json(warning ? { ok: true, warning } : { ok: true });
+    // The browser counts the maximum session length from the server's time.
+    return NextResponse.json(warning ? { ok: true, warning, signedInAt } : { ok: true, signedInAt });
   } catch (error) {
     if (error instanceof JmapAuthVerificationError) {
       return NextResponse.json(
@@ -242,7 +237,7 @@ export async function GET(request: NextRequest) {
     }
 
     const credentials = decryptSession(token);
-    if (!credentials) {
+    if (!credentials || holdsMasterPassword(credentials)) {
       cookieStore.delete(cookieName);
       clearStalwartAuthContextInStore(cookieStore, slot);
       return NextResponse.json({ error: 'Invalid session' }, { status: 401 });
@@ -255,13 +250,12 @@ export async function GET(request: NextRequest) {
       serverUrl: credentials.serverUrl,
       username: credentials.username,
       authHeader: sessionAuthHeader(credentials),
-      signedInAt: age.signedInAt,
     });
 
     // Only return non-sensitive fields. Use PUT to retrieve full credentials.
     const { serverUrl, username } = credentials;
     return NextResponse.json(
-      { serverUrl, username },
+      { serverUrl, username, signedInAt: age.signedInAt },
       { headers: { 'Cache-Control': 'no-store, no-cache, must-revalidate' } },
     );
   } catch (error) {
@@ -309,12 +303,10 @@ export async function PUT(request: NextRequest) {
       serverUrl: credentials.serverUrl,
       username: credentials.username,
       authHeader: sessionAuthHeader(credentials),
-      signedInAt: age.signedInAt,
     });
 
-    // The credentials as they were stored, without the bookkeeping.
-    const { signedInAt: _signedInAt, ...restored } = credentials;
-    return NextResponse.json(restored, {
+    // The browser counts the maximum session length from the server's time.
+    return NextResponse.json({ ...credentials, signedInAt: age.signedInAt }, {
       headers: { 'Cache-Control': 'no-store, no-cache, must-revalidate' },
     });
   } catch (error) {
diff --git a/app/api/auth/sso/complete/route.ts b/app/api/auth/sso/complete/route.ts
index 5ae1dc11c..f79fdb6b9 100644
--- a/app/api/auth/sso/complete/route.ts
+++ b/app/api/auth/sso/complete/route.ts
@@ -106,6 +106,7 @@ export async function POST(request: NextRequest) {
 
     // Exchange code for tokens
     const tokens = await exchangeCodeForTokens(code, codeVerifier, redirectUri, pendingServerId);
+    const signedInAt = Date.now();
 
     // For the mobile handoff flow the tokens are handed back to the app
     // verbatim - we deliberately don't write any cookies on the webmail
@@ -117,7 +118,7 @@ export async function POST(request: NextRequest) {
         const cookieName = refreshTokenCookieName(slot);
         cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
         // A sign-in: the maximum session length counts from here (lib/auth/session-age.ts).
-        writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
+        writeSignInStamp(cookieStore, slot, { signedInAt, refreshToken: tokens.refresh_token });
       }
       const serverCookieName = refreshTokenServerCookieName(slot);
       if (pendingServerId) {
@@ -169,6 +170,7 @@ export async function POST(request: NextRequest) {
     return NextResponse.json({
       access_token: tokens.access_token,
       expires_in: tokens.expires_in,
+      signed_in_at: signedInAt,
     });
   } catch (error) {
     // Clean up pending cookie on any error
diff --git a/app/api/auth/stalwart-context/route.ts b/app/api/auth/stalwart-context/route.ts
index 43c884d1e..fe25532d3 100644
--- a/app/api/auth/stalwart-context/route.ts
+++ b/app/api/auth/stalwart-context/route.ts
@@ -2,8 +2,9 @@ import { NextRequest, NextResponse } from 'next/server';
 import { logger } from '@/lib/logger';
 import { JmapAuthVerificationError, resolveJmapIdentity } from '@/lib/auth/verify-jmap-auth';
 import { cookies } from 'next/headers';
-import { readStalwartAuthContextFromStore, setStalwartAuthContext } from '@/lib/stalwart/auth-context';
+import { setStalwartAuthContext } from '@/lib/stalwart/auth-context';
 import { readSignInStamp, writeSignInStamp } from '@/lib/auth/session-age';
+import { refreshTokenCookieName } from '@/lib/oauth/tokens';
 import { configManager } from '@/lib/admin/config-manager';
 import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { recordLogin } from '@/lib/telemetry/login-tracker';
@@ -103,24 +104,21 @@ export async function POST(request: NextRequest) {
     // Just signed in: an older refusal of this credential no longer applies.
     recordStoredCredentialStatus(authHeader, 200);
 
-    // The context is written again on every renewed token and restore, so
-    // the sign-in time is carried over: from this slot's context for the
-    // same account, else from the token sign-in's stamp, else it is now.
+    // A token sign-in's stamp (lib/auth/session-age.ts) knows no account:
+    // the token routes never see one. The first verified account names it,
+    // so the maximum session length of its domain applies on renewal.
     const cookieStore = await cookies();
-    const previous = readStalwartAuthContextFromStore(cookieStore, slot);
-    const stamp = readSignInStamp(cookieStore, slot);
-    const ownStamp = stamp && (!stamp.account || stamp.account === accountName) ? stamp : null;
-    const samePrevious = previous && previous.serverUrl === normalizedServerUrl && previous.username === username ? previous : null;
-    const signedInAt = samePrevious?.signedInAt ?? ownStamp?.signedInAt ?? Date.now();
-    // The token routes know no account; the first verified one is the stamp's.
-    if (ownStamp && !ownStamp.account) writeSignInStamp(cookieStore, slot, { ...ownStamp, account: accountName });
+    const refreshToken = cookieStore.get(refreshTokenCookieName(slot))?.value;
+    const stamp = refreshToken ? readSignInStamp(cookieStore, slot, refreshToken) : null;
+    if (refreshToken && stamp && !stamp.account) {
+      writeSignInStamp(cookieStore, slot, { signedInAt: stamp.signedInAt, refreshToken, account: accountName });
+    }
 
     await setStalwartAuthContext(slot, {
       serverUrl: normalizedServerUrl,
       username,
       authHeader,
       ...(accountName !== username ? { accountName } : {}),
-      signedInAt,
     });
 
     void recordLogin(username, normalizedServerUrl);
diff --git a/app/api/auth/token/route.ts b/app/api/auth/token/route.ts
index 942514fde..b2ea53c3e 100644
--- a/app/api/auth/token/route.ts
+++ b/app/api/auth/token/route.ts
@@ -34,8 +34,8 @@ import {
   clearSignInStamp,
   isPastMaxAge,
   maxSessionHoursFor,
-  readSignInStamp,
   SESSION_TOO_OLD,
+  tokenSignedInAt,
   writeSignInStamp,
 } from '@/lib/auth/session-age';
 import { clearStalwartAuthContextInStore, readStalwartAuthContextFromStore } from '@/lib/stalwart/auth-context';
@@ -151,18 +151,21 @@ export async function POST(request: NextRequest) {
     const serverId = typeof bodyServerId === 'string' && bodyServerId ? bodyServerId : null;
 
     const tokens = await exchangeCodeForTokens(code, code_verifier, redirect_uri, serverId);
+    // A sign-in: the maximum session length counts from here, on the server
+    // and, from this answer, in the browser.
+    const signedInAt = Date.now();
 
     const response = NextResponse.json({
       access_token: tokens.access_token,
       expires_in: tokens.expires_in,
+      signed_in_at: signedInAt,
     });
 
     const cookieStore = await cookies();
     if (tokens.refresh_token) {
       const cookieName = refreshTokenCookieName(slot);
       cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
-      // A sign-in: the maximum session length counts from here.
-      writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
+      writeSignInStamp(cookieStore, slot, { signedInAt, refreshToken: tokens.refresh_token });
     }
     const cachedAccessToken = cacheAccessToken(cookieStore, slot, tokens.access_token, tokens.expires_in || 3600, [tokens.refresh_token, serverId]);
     storeIdToken(cookieStore, slot, tokens.id_token, request.nextUrl.basePath, [tokens.refresh_token, cachedAccessToken, serverId]);
@@ -201,20 +204,23 @@ export async function PUT(request: NextRequest) {
     }
 
     // The organisation's maximum session length (lib/auth/session-age.ts).
-    // A refresh token from before sign-in times were recorded counts as
-    // signed in now.
-    const stamp = readSignInStamp(cookieStore, slot);
-    if (!stamp) {
-      writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
-    } else {
+    // A refresh token without a matching stamp (from before stamps, or one
+    // whose stamp was deleted) counts from the fixed time the server first
+    // enforced the limits, never from now.
+    const age = await tokenSignedInAt(cookieStore, slot, refreshToken);
+    if (age) {
       const context = readStalwartAuthContextFromStore(cookieStore, slot);
-      const maxHours = await maxSessionHoursFor(stamp.account ?? context?.accountName ?? context?.username ?? null);
-      if (isPastMaxAge(stamp.signedInAt, maxHours)) {
+      const account = age.stamp?.account ?? context?.accountName ?? context?.username ?? null;
+      const maxHours = await maxSessionHoursFor(account);
+      if (isPastMaxAge(age.signedInAt, maxHours)) {
         await signOutSlot(cookieStore, slot, request.nextUrl.basePath, false);
         clearStalwartAuthContextInStore(cookieStore, slot);
         return NextResponse.json({ error: SESSION_TOO_OLD, maxHours }, { status: 401 });
       }
+      // Bind the time to this token from now on.
+      if (!age.stamp) writeSignInStamp(cookieStore, slot, { signedInAt: age.signedInAt, refreshToken, ...(account ? { account } : {}) });
     }
+    const signedIn = age ? { signed_in_at: age.signedInAt } : {};
 
     // A session restore calls this to get its token back, not because the
     // current one expired. Serving the cached token avoids spending a refresh
@@ -231,6 +237,7 @@ export async function PUT(request: NextRequest) {
         return NextResponse.json({
           access_token: cached.accessToken,
           expires_in: cached.expiresIn,
+          ...signedIn,
         });
       }
     }
@@ -279,6 +286,14 @@ export async function PUT(request: NextRequest) {
 
     if (tokens.refresh_token) {
       cookieStore.set(cookieName, tokens.refresh_token, getCookieOptions());
+      // The stamp follows the rotated token, with the same sign-in time and lifetime.
+      if (age) {
+        writeSignInStamp(cookieStore, slot, {
+          signedInAt: age.signedInAt,
+          refreshToken: tokens.refresh_token,
+          ...(age.stamp?.account ? { account: age.stamp.account } : {}),
+        });
+      }
     }
 
     const expiresIn = tokens.expires_in || 3600;
@@ -292,6 +307,7 @@ export async function PUT(request: NextRequest) {
     return NextResponse.json({
       access_token: tokens.access_token,
       expires_in: expiresIn,
+      ...signedIn,
     });
   } catch (error) {
     logger.error('Token refresh error', { error: error instanceof Error ? error.message : 'Unknown error' });
diff --git a/app/api/auth/totp-token-exchange/route.ts b/app/api/auth/totp-token-exchange/route.ts
index bdceadbf7..9b61b428f 100644
--- a/app/api/auth/totp-token-exchange/route.ts
+++ b/app/api/auth/totp-token-exchange/route.ts
@@ -209,13 +209,15 @@ async function storeAndRespond(
   rememberMe: boolean,
 ): Promise<NextResponse> {
   const cookieStore = await cookies();
+  // A sign-in: the maximum session length counts from here (lib/auth/session-age.ts).
+  const signedInAt = Date.now();
   if (tokens.refresh_token) {
     const cookieName = refreshTokenCookieName(slot);
     // Without "Remember me" the sign-in ends with the browser session.
     const { maxAge: _maxAge, ...sessionOptions } = getCookieOptions();
-    cookieStore.set(cookieName, tokens.refresh_token, rememberMe ? getCookieOptions() : sessionOptions);
-    // A sign-in: the maximum session length counts from here (lib/auth/session-age.ts).
-    writeSignInStamp(cookieStore, slot, { signedInAt: Date.now() });
+    const options = rememberMe ? getCookieOptions() : sessionOptions;
+    cookieStore.set(cookieName, tokens.refresh_token, options);
+    writeSignInStamp(cookieStore, slot, { signedInAt, refreshToken: tokens.refresh_token }, options);
   }
   const serverCookieName = refreshTokenServerCookieName(slot);
   if (serverId) {
@@ -228,5 +230,6 @@ async function storeAndRespond(
     access_token: tokens.access_token,
     expires_in: tokens.expires_in || 3600,
     has_refresh_token: !!tokens.refresh_token,
+    signed_in_at: signedInAt,
   });
 }
diff --git a/lib/__tests__/auth-crypto.test.ts b/lib/__tests__/auth-crypto.test.ts
index e30d869b5..5d296c0e1 100644
--- a/lib/__tests__/auth-crypto.test.ts
+++ b/lib/__tests__/auth-crypto.test.ts
@@ -36,8 +36,8 @@ describe('encryptSession / decryptSession', () => {
   });
 
   it('produces a base64 token with a random IV (two encrypts differ, both decrypt equal)', () => {
-    const a = encryptSession('https://x', 'u', 'p');
-    const b = encryptSession('https://x', 'u', 'p');
+    const a = encryptSession('https://x', 'u', 'p', 1000);
+    const b = encryptSession('https://x', 'u', 'p', 1000);
     expect(a).not.toBe(b);
     expect(Buffer.from(a, 'base64').toString('base64')).toBe(a); // valid base64
     expect(decryptSession(a)).toEqual(decryptSession(b));
diff --git a/lib/__tests__/impersonation-session.test.ts b/lib/__tests__/impersonation-session.test.ts
index b70abd176..aa44a2171 100644
--- a/lib/__tests__/impersonation-session.test.ts
+++ b/lib/__tests__/impersonation-session.test.ts
@@ -110,7 +110,7 @@ describe('impersonation handoff', () => {
     const put = await readSession();
     expect(put.status).toBe(200);
     const body = await put.json();
-    expect(body).toEqual({ serverUrl: 'https://mail.example.org', username: 'alice@example.org', password: APP_SECRET });
+    expect(body).toEqual({ serverUrl: 'https://mail.example.org', username: 'alice@example.org', password: APP_SECRET, signedInAt: expect.any(Number) });
     expect(JSON.stringify(body)).not.toContain(MASTER_PASSWORD);
     expect([...jar.values()].join()).not.toContain(MASTER_PASSWORD);
   });
diff --git a/lib/__tests__/session-max-age-routes.test.ts b/lib/__tests__/session-max-age-routes.test.ts
index 5b3dd23b8..8048e2a28 100644
--- a/lib/__tests__/session-max-age-routes.test.ts
+++ b/lib/__tests__/session-max-age-routes.test.ts
@@ -1,14 +1,19 @@
 // @vitest-environment node
 import { createCipheriv, createHash, randomBytes } from 'node:crypto';
-import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { mkdtempSync, rmSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import path from 'node:path';
+import { afterAll, afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { NextRequest } from 'next/server';
 import type { SettingsPolicy } from '@/lib/admin/types';
 
 /**
  * The organisation's maximum session length on the server: the routes that
  * restore a remembered sign-in or renew a provider token refuse a session
- * older than `maxHours` of the account's domain scopes, and a cookie from
- * before sign-in times were recorded counts as signed in when first seen.
+ * older than `maxHours` of the account's domain scopes. A cookie or token
+ * without a sign-in time counts from one fixed time, when the server first
+ * enforced the limits, so deleting the stamp or keeping an old cookie never
+ * makes a session younger.
  */
 
 const SECRET = 's'.repeat(64);
@@ -43,7 +48,6 @@ vi.mock('@/lib/auth/verify-jmap-auth', () => ({
   resolveJmapIdentity: async (serverUrl: string, _auth: string, username: string) => ({ serverUrl, accountName: username }),
 }));
 
-const tokenFetch = vi.fn();
 vi.mock('@/lib/oauth/token-exchange', () => ({
   exchangeCodeForTokens: async () => ({ access_token: 'first-access', refresh_token: 'refresh-1', expires_in: 300 }),
   buildOAuthParams: (params: Record<string, string>) => new URLSearchParams(params),
@@ -54,8 +58,12 @@ vi.mock('@/lib/oauth/token-exchange', () => ({
   DEFAULT_CLIENT_ID: 'bulwark-webmail',
 }));
 
-const { encryptSession, decryptSession, decryptPayload } = await import('@/lib/auth/crypto');
-const { readSignInStamp, writeSignInStamp } = await import('@/lib/auth/session-age');
+const stateDir = mkdtempSync(path.join(tmpdir(), 'bw-session-age-'));
+process.env.ADMIN_STATE_DIR = stateDir;
+afterAll(() => rmSync(stateDir, { recursive: true, force: true }));
+
+const { encryptSession, decryptSession } = await import('@/lib/auth/crypto');
+const { readSignInStamp, writeSignInStamp, limitsEnforcedSince, resetLimitsEnforcedSince } = await import('@/lib/auth/session-age');
 const sessionRoute = await import('@/app/api/auth/session/route');
 const tokenRoute = await import('@/app/api/auth/token/route');
 const contextRoute = await import('@/app/api/auth/stalwart-context/route');
@@ -82,19 +90,33 @@ function legacySessionCookie(username: string, password: string): string {
   return Buffer.concat([iv, cipher.getAuthTag(), encrypted]).toString('base64');
 }
 
+const store = async () => (await (await import('next/headers')).cookies()) as never;
 const restore = () => sessionRoute.PUT(new NextRequest('https://webmail.example/api/auth/session', { method: 'PUT', headers: SAME_ORIGIN }));
 const peek = () => sessionRoute.GET(new NextRequest('https://webmail.example/api/auth/session'));
-const refresh = () => tokenRoute.PUT(new NextRequest('https://webmail.example/api/auth/token?slot=0', { method: 'PUT', headers: SAME_ORIGIN }));
-const context = () => decryptPayload(jar.get('jmap_stalwart_ctx')!, 'session-context');
+const refresh = () => tokenRoute.PUT(new NextRequest('https://webmail.example/api/auth/token?slot=0&force=true', { method: 'PUT', headers: SAME_ORIGIN }));
+const signInWithProvider = () => tokenRoute.POST(new NextRequest('https://webmail.example/api/auth/token?slot=0', {
+  method: 'POST',
+  headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
+  body: JSON.stringify({ code: 'c', code_verifier: 'v', redirect_uri: 'https://webmail.example/auth/callback', slot: 0 }),
+}));
 
-beforeEach(() => {
+let issued = 0;
+const tokenFetch = vi.fn();
+
+beforeEach(async () => {
   jar.clear();
+  rmSync(path.join(stateDir, 'session-limits-since'), { force: true });
+  resetLimitsEnforcedSince();
   vi.useFakeTimers({ toFake: ['Date'] });
   vi.setSystemTime(NOW);
+  // The server enforces the limits from NOW on.
+  expect(await limitsEnforcedSince()).toBe(NOW);
   policy = policyWithDomainMax(8);
+  issued = 0;
   tokenFetch.mockReset();
   tokenFetch.mockImplementation(async (url: string) => {
-    if (url.endsWith('/token')) return Response.json({ access_token: 'new-access', expires_in: 300 });
+    // The provider rotates the refresh token on every renewal.
+    if (url.endsWith('/token')) return Response.json({ access_token: 'new-access', refresh_token: `rotated-${++issued}`, expires_in: 300 });
     return new Response(null, { status: 200 });
   });
   vi.stubGlobal('fetch', tokenFetch);
@@ -105,24 +127,31 @@ afterEach(() => {
   vi.unstubAllGlobals();
 });
 
+describe('the time the limits were first enforced', () => {
+  it('is recorded once and kept across restarts', async () => {
+    vi.setSystemTime(NOW + 5 * HOUR);
+    resetLimitsEnforcedSince();
+    expect(await limitsEnforcedSince()).toBe(NOW);
+  });
+});
+
 describe('remembered sessions (/api/auth/session)', () => {
-  it('records the sign-in time when the session is remembered', async () => {
+  it('records the sign-in time when the session is remembered, and answers with it', async () => {
     const res = await sessionRoute.POST(new NextRequest('https://webmail.example/api/auth/session?slot=0', {
       method: 'POST',
       headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
       body: JSON.stringify({ serverUrl: SERVER, username: 'org-alice@example.org', password: 'pw' }),
     }));
     expect(res.status).toBe(200);
+    expect(await res.json()).toEqual({ ok: true, signedInAt: NOW });
     expect(decryptSession(jar.get('jmap_session')!)?.signedInAt).toBe(NOW);
-    expect(context()?.signedInAt).toBe(NOW);
   });
 
-  it('restores a session younger than the maximum, without the sign-in time in the answer', async () => {
+  it('restores a session younger than the maximum and says when it signed in', async () => {
     jar.set('jmap_session', encryptSession(SERVER, 'org-alice@example.org', 'pw', NOW - 7 * HOUR));
     const res = await restore();
     expect(res.status).toBe(200);
-    expect(await res.json()).toEqual({ serverUrl: SERVER, username: 'org-alice@example.org', password: 'pw' });
-    expect(context()?.signedInAt).toBe(NOW - 7 * HOUR);
+    expect(await res.json()).toEqual({ serverUrl: SERVER, username: 'org-alice@example.org', password: 'pw', signedInAt: NOW - 7 * HOUR });
   });
 
   it('refuses a session older than its domain allows and clears its cookies', async () => {
@@ -150,41 +179,46 @@ describe('remembered sessions (/api/auth/session)', () => {
     expect((await restore()).status).toBe(401);
   });
 
-  it('counts a cookie without a sign-in time as signed in when first seen', async () => {
-    jar.set('jmap_session', legacySessionCookie('org-alice@example.org', 'pw'));
-
-    const first = await restore();
-    expect(first.status).toBe(200);
-    expect(await first.json()).toEqual({ serverUrl: SERVER, username: 'org-alice@example.org', password: 'pw' });
-    expect(decryptSession(jar.get('jmap_session')!)?.signedInAt).toBe(NOW);
+  it('counts a cookie without a sign-in time from the fixed time, without rewriting it', async () => {
+    const legacy = legacySessionCookie('org-alice@example.org', 'pw');
+    jar.set('jmap_session', legacy);
 
     vi.setSystemTime(NOW + 7 * HOUR);
-    expect((await restore()).status).toBe(200);
+    const first = await restore();
+    expect(first.status).toBe(200);
+    expect((await first.json()).signedInAt).toBe(NOW);
+    expect(jar.get('jmap_session')).toBe(legacy);
+
     vi.setSystemTime(NOW + 8 * HOUR);
     expect((await restore()).status).toBe(401);
   });
+
+  it('refuses a kept copy of an old cookie replayed later', async () => {
+    const legacy = legacySessionCookie('org-alice@example.org', 'pw');
+    // First seen only now, nine hours after the server started enforcing.
+    vi.setSystemTime(NOW + 9 * HOUR);
+    jar.set('jmap_session', legacy);
+    expect((await restore()).status).toBe(401);
+    jar.set('jmap_session', legacy);
+    expect((await peek()).status).toBe(401);
+  });
 });
 
 describe('provider tokens (/api/auth/token)', () => {
-  it('records the sign-in time when a provider sign-in stores its refresh token', async () => {
-    // A leftover stamp from an earlier sign-in in this slot.
-    const store = (await (await import('next/headers')).cookies()) as never;
-    writeSignInStamp(store, 0, { signedInAt: NOW - 20 * HOUR, account: 'org-bob@example.org' });
-
-    const res = await tokenRoute.POST(new NextRequest('https://webmail.example/api/auth/token?slot=0', {
-      method: 'POST',
-      headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
-      body: JSON.stringify({ code: 'c', code_verifier: 'v', redirect_uri: 'https://webmail.example/auth/callback', slot: 0 }),
-    }));
+  it('records the sign-in time with the refresh token when a provider sign-in stores it', async () => {
+    const res = await signInWithProvider();
     expect(res.status).toBe(200);
-    expect(readSignInStamp(store, 0)).toEqual({ signedInAt: NOW });
+    expect((await res.json()).signed_in_at).toBe(NOW);
+    expect(readSignInStamp(await store(), 0, 'refresh-1')).toMatchObject({ signedInAt: NOW });
+    // The stamp belongs to that token only.
+    expect(readSignInStamp(await store(), 0, 'another-token')).toBeNull();
   });
 
   it('refuses to renew a token sign-in older than the maximum and revokes it', async () => {
     jar.set('jmap_rt', 'refresh-1');
-    const store = (await (await import('next/headers')).cookies()) as never;
-    writeSignInStamp(store, 0, { signedInAt: NOW - 8 * HOUR, account: 'org-alice@example.org' });
+    writeSignInStamp(await store(), 0, { signedInAt: NOW + HOUR, refreshToken: 'refresh-1', account: 'org-alice@example.org' });
 
+    vi.setSystemTime(NOW + 9 * HOUR);
     const res = await refresh();
     expect(res.status).toBe(401);
     expect(await res.json()).toEqual({ error: 'session_expired', maxHours: 8 });
@@ -194,47 +228,70 @@ describe('provider tokens (/api/auth/token)', () => {
     expect(tokenFetch).not.toHaveBeenCalledWith('https://id.example.org/token', expect.anything());
   });
 
-  it('renews a token sign-in younger than the maximum', async () => {
-    jar.set('jmap_rt', 'refresh-1');
-    const store = (await (await import('next/headers')).cookies()) as never;
-    writeSignInStamp(store, 0, { signedInAt: NOW - 7 * HOUR, account: 'org-alice@example.org' });
+  it('renews a younger one, and the stamp follows the rotated token with the same time', async () => {
+    await signInWithProvider();
+    vi.setSystemTime(NOW + 3 * HOUR);
 
     const res = await refresh();
     expect(res.status).toBe(200);
-    expect((await res.json()).access_token).toBe('new-access');
+    expect(await res.json()).toMatchObject({ access_token: 'new-access', signed_in_at: NOW });
+    expect(jar.get('jmap_rt')).toBe('rotated-1');
+    expect(readSignInStamp(await store(), 0, 'rotated-1')).toMatchObject({ signedInAt: NOW });
+
+    vi.setSystemTime(NOW + 7 * HOUR);
+    expect((await refresh()).status).toBe(200);
+    vi.setSystemTime(NOW + 8 * HOUR);
+    expect((await refresh()).status).toBe(401);
   });
 
-  it('counts a refresh token without a sign-in time as signed in when first seen', async () => {
-    jar.set('jmap_rt', 'refresh-1');
+  it('counts a refresh token without a stamp from the fixed time and binds that time to it', async () => {
+    jar.set('jmap_rt', 'refresh-legacy');
+    vi.setSystemTime(NOW + HOUR);
 
+    const res = await refresh();
+    expect(res.status).toBe(200);
+    expect((await res.json()).signed_in_at).toBe(NOW);
+    expect(readSignInStamp(await store(), 0, 'rotated-1')).toMatchObject({ signedInAt: NOW });
+
+    vi.setSystemTime(NOW + 8 * HOUR);
+    expect((await refresh()).status).toBe(401);
+  });
+
+  it('does not make a session younger when its stamp is deleted', async () => {
+    vi.setSystemTime(NOW + 5 * HOUR);
+    await signInWithProvider();
+    jar.delete('jmap_signed_in');
+
+    vi.setSystemTime(NOW + 8 * HOUR);
+    const res = await refresh();
+    expect(res.status).toBe(401);
+  });
+
+  it('applies the domain of the account the stamp names', async () => {
+    jar.set('jmap_rt', 'refresh-1');
+    writeSignInStamp(await store(), 0, { signedInAt: NOW, refreshToken: 'refresh-1', account: 'org-dave@org-two.test' });
+    vi.setSystemTime(NOW + 100 * HOUR);
     expect((await refresh()).status).toBe(200);
-    const store = (await (await import('next/headers')).cookies()) as never;
-    expect(readSignInStamp(store, 0)).toEqual({ signedInAt: NOW });
+  });
+
+  it('does not take a stamp written for another refresh token', async () => {
+    jar.set('jmap_rt', 'refresh-2');
+    writeSignInStamp(await store(), 0, { signedInAt: NOW + 7 * HOUR, refreshToken: 'refresh-1' });
+
+    vi.setSystemTime(NOW + 8 * HOUR);
+    expect((await refresh()).status).toBe(401);
   });
 });
 
 describe('the auth context (/api/auth/stalwart-context)', () => {
-  const post = () => contextRoute.POST(new NextRequest('https://webmail.example/api/auth/stalwart-context', {
-    method: 'POST',
-    headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
-    body: JSON.stringify({ serverUrl: SERVER, username: 'org-alice@example.org', authHeader: 'Bearer new', slot: 0 }),
-  }));
-
-  it('keeps the sign-in time when a renewed token is written again', async () => {
-    jar.set('jmap_session', encryptSession(SERVER, 'org-alice@example.org', 'pw', NOW - 3 * HOUR));
-    await restore();
-    vi.setSystemTime(NOW + HOUR);
-
-    expect((await post()).status).toBe(200);
-    expect(context()?.signedInAt).toBe(NOW - 3 * HOUR);
-  });
-
-  it("takes a token sign-in's time from its stamp and names the account on it", async () => {
-    const store = (await (await import('next/headers')).cookies()) as never;
-    writeSignInStamp(store, 0, { signedInAt: NOW - 2 * HOUR });
-
-    expect((await post()).status).toBe(200);
-    expect(context()?.signedInAt).toBe(NOW - 2 * HOUR);
-    expect(readSignInStamp(store, 0)).toEqual({ signedInAt: NOW - 2 * HOUR, account: 'org-alice@example.org' });
+  it("names the account on a token sign-in's stamp", async () => {
+    await signInWithProvider();
+    const res = await contextRoute.POST(new NextRequest('https://webmail.example/api/auth/stalwart-context', {
+      method: 'POST',
+      headers: { ...SAME_ORIGIN, 'content-type': 'application/json' },
+      body: JSON.stringify({ serverUrl: SERVER, username: 'org-alice@example.org', authHeader: 'Bearer new', slot: 0 }),
+    }));
+    expect(res.status).toBe(200);
+    expect(readSignInStamp(await store(), 0, 'refresh-1')).toMatchObject({ signedInAt: NOW, account: 'org-alice@example.org' });
   });
 });
diff --git a/lib/auth/session-age.ts b/lib/auth/session-age.ts
index e5689e889..ba7f97e8a 100644
--- a/lib/auth/session-age.ts
+++ b/lib/auth/session-age.ts
@@ -8,41 +8,58 @@
  * back: the routes that restore a remembered sign-in (/api/auth/session) or
  * renew a provider token (/api/auth/token) refuse it and clear its cookies.
  *
- * The sign-in time travels in the remembered-session cookie (`signedInAt`
- * in lib/auth/crypto.ts), in the auth-context cookie, and for token
- * sign-ins in a small cookie of its own written when the refresh token is
- * (the auth-context cookie ends with the browser, the refresh token lasts
- * 30 days). A cookie written before this existed has no sign-in time: it
- * counts as signed in when the server first sees it after the upgrade,
- * which then writes the time into it.
+ * The sign-in time travels in the remembered-session cookie (`signedInAt` in
+ * lib/auth/crypto.ts) and, for token sign-ins, in a stamp cookie of its own
+ * that is bound to the refresh token by a hash and rewritten with it on
+ * every rotation. A cookie or token without a sign-in time (one written
+ * before this existed, or a stamp that was deleted or doesn't match the
+ * token) counts as signed in at one fixed time: when the server first
+ * enforced the limits (limitsEnforcedSince). Deleting the stamp therefore
+ * never makes a session younger.
  *
  * The server knows an account's address, not its groups, so only Everyone
  * and the domain scopes count here; a group scope's value is enforced by the
  * client alone.
  */
 
+import { createHash } from 'node:crypto';
+import { readFile, writeFile } from 'node:fs/promises';
 import type { cookies } from 'next/headers';
 import { configManager } from '@/lib/admin/config-manager';
+import { ensureStateDir, getStatePath } from '@/lib/admin/paths';
 import { resolvePolicy } from '@/lib/admin/policy-scopes';
 import { decryptPayload, encryptPayload } from '@/lib/auth/crypto';
+import { hasSessionSecret } from '@/lib/auth/session-secret';
 import { getCookieOptions } from '@/lib/oauth/cookie-config';
 import { logger } from '@/lib/logger';
 
+export { SESSION_TOO_OLD } from '@/lib/session-policy/constants';
+
 type CookieStore = Awaited<ReturnType<typeof cookies>>;
 
 const HOUR_MS = 60 * 60 * 1000;
 
-/** What a route answers when it refuses a session that is too old. */
-export const SESSION_TOO_OLD = 'session_expired';
+const positiveHours = (value: unknown): number => (typeof value === 'number' && value > 0 ? value : 0);
 
-/** The maximum session length (hours, 0 = none) for an account's address. */
+/**
+ * The maximum session length (hours, 0 = none) for an account's address.
+ * Without an address (a token sign-in whose stamp names no account, which
+ * includes one whose stamp was deleted) the strictest value of Everyone and
+ * every domain entry applies, so losing the account never loosens the limit.
+ */
 export async function maxSessionHoursFor(account: string | null | undefined): Promise<number> {
   try {
     await configManager.ensureLoaded?.();
+    const raw = configManager.getPolicy();
     const email = account?.trim().toLowerCase() ?? '';
-    const policy = resolvePolicy(configManager.getPolicy(), email ? { email, groups: [] } : null).policy;
-    const hours = policy.organisation?.session?.maxHours;
-    return typeof hours === 'number' && hours > 0 ? hours : 0;
+    if (email) return positiveHours(resolvePolicy(raw, { email, groups: [] }).policy.organisation?.session?.maxHours);
+    const values = [
+      positiveHours(resolvePolicy(raw, null).policy.organisation?.session?.maxHours),
+      ...(raw.scopes ?? [])
+        .filter((scope) => (scope.match?.domains?.length ?? 0) > 0)
+        .map((scope) => positiveHours(scope.policy?.organisation?.session?.maxHours)),
+    ].filter((hours) => hours > 0);
+    return values.length > 0 ? Math.min(...values) : 0;
   } catch (error) {
     logger.warn('Could not read the maximum session length', { error: error instanceof Error ? error.message : 'Unknown error' });
     return 0;
@@ -50,47 +67,120 @@ export async function maxSessionHoursFor(account: string | null | undefined): Pr
 }
 
 /** Whether a session signed in at `signedInAt` has outlived `maxHours`. */
-export function isPastMaxAge(signedInAt: number | undefined, maxHours: number, now = Date.now()): boolean {
-  if (!maxHours || typeof signedInAt !== 'number') return false;
+export function isPastMaxAge(signedInAt: number, maxHours: number, now = Date.now()): boolean {
+  if (!maxHours) return false;
   return now - signedInAt >= maxHours * HOUR_MS;
 }
 
-export function validSignedInAt(value: unknown): number | undefined {
-  return typeof value === 'number' && Number.isFinite(value) && value > 0 ? value : undefined;
+// ── The fixed time for sessions without a sign-in time ─────────────────────
+
+const ENFORCED_SINCE_FILE = 'session-limits-since';
+const PROCESS_START = Date.now() - Math.round(process.uptime() * 1000);
+let enforcedSince: Promise<number> | null = null;
+
+function parseTime(text: string): number | null {
+  const value = Number(text.trim());
+  return Number.isFinite(value) && value > 0 ? value : null;
+}
+
+async function readRecorded(path: string): Promise<number | null> {
+  try {
+    return parseTime(await readFile(path, 'utf8'));
+  } catch {
+    return null;
+  }
 }
 
 /**
- * The sign-in time of a token sign-in, kept as long as its refresh token.
- * `account` is filled in once the auth context names the account.
+ * When this server first enforced the session limits, recorded once in the
+ * admin state dir. Falls back to the process start when the file can't be
+ * written (a read-only state dir), which restarts the count on a restart.
+ */
+export function limitsEnforcedSince(): Promise<number> {
+  enforcedSince ??= (async () => {
+    const path = getStatePath(ENFORCED_SINCE_FILE);
+    const recorded = await readRecorded(path);
+    if (recorded) return recorded;
+    const now = Date.now();
+    try {
+      await ensureStateDir();
+      // 'wx': a second process starting at the same moment keeps the first one's time.
+      await writeFile(path, String(now), { flag: 'wx' });
+      return now;
+    } catch {
+      const other = await readRecorded(path);
+      if (other) return other;
+      logger.warn('Could not record when the session limits were first enforced; counting from the process start');
+      return PROCESS_START;
+    }
+  })();
+  return enforcedSince;
+}
+
+/** Test helper. */
+export function resetLimitsEnforcedSince(): void {
+  enforcedSince = null;
+}
+
+// ── The sign-in stamp of a token sign-in ───────────────────────────────────
+
+/**
+ * The sign-in time of a token sign-in, bound to the refresh token it was
+ * written with. `account` is filled in once the auth context names it.
  */
 export interface SignInStamp {
   signedInAt: number;
+  tokenHash: string;
   account?: string;
 }
 
+export function refreshTokenHash(refreshToken: string): string {
+  return createHash('sha256').update(refreshToken).digest('base64url');
+}
+
 export function signInStampCookieName(slot: number): string {
   return slot === 0 ? 'jmap_signed_in' : `jmap_signed_in_${slot}`;
 }
 
-export function readSignInStamp(cookieStore: CookieStore, slot: number): SignInStamp | null {
+/** The slot's stamp, or null when it is missing, unreadable, or written for another refresh token. */
+export function readSignInStamp(cookieStore: CookieStore, slot: number, refreshToken: string): SignInStamp | null {
   const value = cookieStore.get(signInStampCookieName(slot))?.value;
   if (!value) return null;
   try {
     const payload = decryptPayload(value, 'session-age');
-    const signedInAt = validSignedInAt(payload?.signedInAt);
-    if (!signedInAt) return null;
-    return typeof payload?.account === 'string' ? { signedInAt, account: payload.account } : { signedInAt };
+    const signedInAt = payload?.signedInAt;
+    if (typeof signedInAt !== 'number' || !Number.isFinite(signedInAt)) return null;
+    if (payload?.tokenHash !== refreshTokenHash(refreshToken)) return null;
+    const account = typeof payload.account === 'string' ? payload.account : undefined;
+    return { signedInAt, tokenHash: payload.tokenHash, ...(account ? { account } : {}) };
   } catch {
     return null;
   }
 }
 
-/** Without a session secret nothing can be sealed; the client's timer still applies. */
-export function writeSignInStamp(cookieStore: CookieStore, slot: number, stamp: SignInStamp): void {
+let missingSecretLogged = false;
+
+/**
+ * Written whenever the refresh token cookie is, with that cookie's options.
+ * Without a session secret nothing can be sealed; the client's timer still applies.
+ */
+export function writeSignInStamp(
+  cookieStore: CookieStore,
+  slot: number,
+  stamp: { signedInAt: number; refreshToken: string; account?: string },
+  options: Record<string, unknown> = getCookieOptions(),
+): void {
+  if (!hasSessionSecret()) {
+    if (!missingSecretLogged) {
+      missingSecretLogged = true;
+      logger.warn('No session secret: the maximum session length is enforced by the browser only');
+    }
+    return;
+  }
   try {
-    const payload: Record<string, unknown> = { signedInAt: stamp.signedInAt };
+    const payload: Record<string, unknown> = { signedInAt: stamp.signedInAt, tokenHash: refreshTokenHash(stamp.refreshToken) };
     if (stamp.account) payload.account = stamp.account;
-    cookieStore.set(signInStampCookieName(slot), encryptPayload(payload, 'session-age'), getCookieOptions());
+    cookieStore.set(signInStampCookieName(slot), encryptPayload(payload, 'session-age'), options);
   } catch (error) {
     logger.warn('Could not record the sign-in time', { error: error instanceof Error ? error.message : 'Unknown error' });
   }
@@ -100,3 +190,18 @@ export function clearSignInStamp(cookieStore: CookieStore, slot: number): void {
   const name = signInStampCookieName(slot);
   if (cookieStore.get(name)) cookieStore.delete(name);
 }
+
+/**
+ * When a token sign-in happened: its stamp's time, or the fixed time when
+ * there is no stamp for this refresh token. Null without a session secret,
+ * when the server can't keep sign-in times at all.
+ */
+export async function tokenSignedInAt(
+  cookieStore: CookieStore,
+  slot: number,
+  refreshToken: string,
+): Promise<{ signedInAt: number; stamp: SignInStamp | null } | null> {
+  if (!hasSessionSecret()) return null;
+  const stamp = readSignInStamp(cookieStore, slot, refreshToken);
+  return { signedInAt: stamp?.signedInAt ?? await limitsEnforcedSince(), stamp };
+}
diff --git a/lib/session-policy/constants.ts b/lib/session-policy/constants.ts
new file mode 100644
index 000000000..dab0c03a7
--- /dev/null
+++ b/lib/session-policy/constants.ts
@@ -0,0 +1,6 @@
+/**
+ * What the sign-in routes answer (`error`) when they refuse a session that
+ * reached the organisation's maximum length. The server (lib/auth/session-age.ts)
+ * and the browser (stores/auth-store.ts) both read it from here.
+ */
+export const SESSION_TOO_OLD = 'session_expired';
diff --git a/lib/stalwart/auth-context.ts b/lib/stalwart/auth-context.ts
index 3a34f1901..7d22b3f63 100644
--- a/lib/stalwart/auth-context.ts
+++ b/lib/stalwart/auth-context.ts
@@ -14,12 +14,6 @@ export interface StalwartAuthContext {
    * data such as synced settings is keyed on this.
    */
   accountName?: string;
-  /**
-   * When the user signed in (ms), for the organisation's maximum session
-   * length (lib/auth/session-age.ts). Carried over when the context is
-   * written again for the same account (a renewed token, a restore).
-   */
-  signedInAt?: number;
 }
 
 type CookieStore = Awaited<ReturnType<typeof cookies>>;
@@ -45,8 +39,7 @@ function isValidContext(payload: unknown): payload is StalwartAuthContext {
   return typeof candidate.serverUrl === 'string'
     && typeof candidate.username === 'string'
     && typeof candidate.authHeader === 'string'
-    && (candidate.accountName === undefined || typeof candidate.accountName === 'string')
-    && (candidate.signedInAt === undefined || typeof candidate.signedInAt === 'number');
+    && (candidate.accountName === undefined || typeof candidate.accountName === 'string');
 }
 
 function getSessionCookieOptions() {
@@ -66,14 +59,8 @@ export function readStalwartAuthContextFromStore(
 
   // Hand back exactly the declared shape: the envelope's own bookkeeping
   // stays in the envelope rather than riding along into the credentials.
-  const { serverUrl, username, authHeader, accountName, signedInAt } = payload;
-  return {
-    serverUrl,
-    username,
-    authHeader,
-    ...(accountName ? { accountName } : {}),
-    ...(typeof signedInAt === 'number' ? { signedInAt } : {}),
-  };
+  const { serverUrl, username, authHeader, accountName } = payload;
+  return accountName ? { serverUrl, username, authHeader, accountName } : { serverUrl, username, authHeader };
 }
 
 export function setStalwartAuthContextInStore(