diff --git a/apps/api/tests/routes/admin.test.ts b/apps/api/tests/routes/admin.test.ts
index 81d5248..979e92e 100644
--- a/apps/api/tests/routes/admin.test.ts
+++ b/apps/api/tests/routes/admin.test.ts
@@ -30,7 +30,7 @@ import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import { randomUUID } from 'node:crypto';
 import { eq } from 'drizzle-orm';
 import { db } from '../../src/db/client.js';
-import { users, memberCredentials, calendars } from '../../src/db/schema.js';
+import { users, memberCredentials, calendars, appConfig } from '../../src/db/schema.js';
 
 // ---------------------------------------------------------------------------
 // CalDAV mock — intercept createFastmailClient so tests do not hit live Fastmail.
@@ -585,3 +585,171 @@ describe('POST /api/me/credential', () => {
     expect(res.status).toBe(200);
   });
 });
+
+// ===========================================================================
+// admin timezone config (Plan 18-02: D-01, D-02, D-03, D-04)
+// ===========================================================================
+
+describe('admin timezone config', () => {
+  // Clean up household_timezone between tests to avoid bleed
+  afterEach(async () => {
+    await db.delete(appConfig).where(eq(appConfig.key, 'household_timezone'));
+  });
+
+  // -------------------------------------------------------------------------
+  // GET /api/admin/config/timezone — access control
+  // -------------------------------------------------------------------------
+
+  it('GET returns 403 for a non-admin authenticated user (T-18-03)', async () => {
+    const nonAdminId = await seedUser('tz-non-admin-get', false);
+    currentDevUserId = nonAdminId;
+    const app = await getApp();
+
+    const res = await app.fetch(jsonRequest('GET', '/api/admin/config/timezone'));
+    expect(res.status).toBe(403);
+  });
+
+  // -------------------------------------------------------------------------
+  // PUT /api/admin/config/timezone — access control
+  // -------------------------------------------------------------------------
+
+  it('PUT returns 403 for a non-admin authenticated user (T-18-03)', async () => {
+    const nonAdminId = await seedUser('tz-non-admin-put', false);
+    currentDevUserId = nonAdminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PUT', '/api/admin/config/timezone', { timezone: 'America/Chicago' }),
+    );
+    expect(res.status).toBe(403);
+  });
+
+  // -------------------------------------------------------------------------
+  // GET /api/admin/config/timezone — admin reads when no row is stored
+  // -------------------------------------------------------------------------
+
+  it('GET as admin with no stored row returns 200 with isExplicitlySet: false and a non-empty timezone', async () => {
+    const adminId = await seedUser('tz-admin-get-default', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(jsonRequest('GET', '/api/admin/config/timezone'));
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { timezone: string; isExplicitlySet: boolean };
+    expect(typeof body.timezone).toBe('string');
+    expect(body.timezone.length).toBeGreaterThan(0);
+    expect(body.isExplicitlySet).toBe(false);
+  });
+
+  // -------------------------------------------------------------------------
+  // PUT then GET round-trip
+  // -------------------------------------------------------------------------
+
+  it('PUT America/Chicago then GET returns that value with isExplicitlySet: true', async () => {
+    const adminId = await seedUser('tz-admin-roundtrip', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const putRes = await app.fetch(
+      jsonRequest('PUT', '/api/admin/config/timezone', { timezone: 'America/Chicago' }),
+    );
+    expect(putRes.status).toBe(200);
+
+    const getRes = await app.fetch(jsonRequest('GET', '/api/admin/config/timezone'));
+    expect(getRes.status).toBe(200);
+    const body = (await getRes.json()) as { timezone: string; isExplicitlySet: boolean };
+    expect(body.timezone).toBe('America/Chicago');
+    expect(body.isExplicitlySet).toBe(true);
+  });
+
+  // -------------------------------------------------------------------------
+  // PUT with UTC must succeed (Pitfall 2)
+  // -------------------------------------------------------------------------
+
+  it('PUT UTC returns 200 (Pitfall 2 — UTC must be accepted)', async () => {
+    const adminId = await seedUser('tz-admin-utc', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PUT', '/api/admin/config/timezone', { timezone: 'UTC' }),
+    );
+    expect(res.status).toBe(200);
+  });
+
+  // -------------------------------------------------------------------------
+  // PUT with invalid IANA string returns 400 and writes nothing (T-18-04)
+  // -------------------------------------------------------------------------
+
+  it('PUT Not/AZone returns 400 and does not store a value (T-18-04)', async () => {
+    const adminId = await seedUser('tz-admin-invalid', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PUT', '/api/admin/config/timezone', { timezone: 'Not/AZone' }),
+    );
+    expect(res.status).toBe(400);
+
+    // Verify no row was written to app_config
+    const [row] = await db
+      .select({ value: appConfig.value })
+      .from(appConfig)
+      .where(eq(appConfig.key, 'household_timezone'))
+      .limit(1);
+    expect(row).toBeUndefined();
+  });
+
+  // -------------------------------------------------------------------------
+  // POST /api/admin/config/timezone/seed — seeds when unset (D-02)
+  // -------------------------------------------------------------------------
+
+  it('POST seed when unset stores the value and returns ok (D-02)', async () => {
+    const adminId = await seedUser('tz-admin-seed-unset', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('POST', '/api/admin/config/timezone/seed', { timezone: 'Europe/London' }),
+    );
+    expect(res.status).toBe(200);
+
+    // Verify the value was stored
+    const [row] = await db
+      .select({ value: appConfig.value })
+      .from(appConfig)
+      .where(eq(appConfig.key, 'household_timezone'))
+      .limit(1);
+    expect(row?.value).toBe('Europe/London');
+  });
+
+  // -------------------------------------------------------------------------
+  // POST /api/admin/config/timezone/seed — no-overwrite when already set (D-03)
+  // -------------------------------------------------------------------------
+
+  it('POST seed when already set does NOT overwrite the existing value (D-03)', async () => {
+    const adminId = await seedUser('tz-admin-seed-overwrite', true);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    // First set a value via PUT
+    const putRes = await app.fetch(
+      jsonRequest('PUT', '/api/admin/config/timezone', { timezone: 'America/Chicago' }),
+    );
+    expect(putRes.status).toBe(200);
+
+    // Now attempt to seed a different value
+    const seedRes = await app.fetch(
+      jsonRequest('POST', '/api/admin/config/timezone/seed', { timezone: 'Asia/Tokyo' }),
+    );
+    expect(seedRes.status).toBe(200);
+
+    // The stored value must still be America/Chicago (no overwrite, D-03)
+    const [row] = await db
+      .select({ value: appConfig.value })
+      .from(appConfig)
+      .where(eq(appConfig.key, 'household_timezone'))
+      .limit(1);
+    expect(row?.value).toBe('America/Chicago');
+  });
+});