diff --git a/apps/api/src/routes/admin.ts b/apps/api/src/routes/admin.ts
index c68f354..797f131 100644
--- a/apps/api/src/routes/admin.ts
+++ b/apps/api/src/routes/admin.ts
@@ -25,8 +25,9 @@ import { zValidator } from '@hono/zod-validator';
 import { z } from 'zod';
 import { eq } from 'drizzle-orm';
 import { db } from '../db/client.js';
-import { users, memberCredentials, calendars } from '../db/schema.js';
+import { users, memberCredentials, calendars, appConfig } from '../db/schema.js';
 import { requireAdmin } from '../lib/requireAdmin.js';
+import { isValidIanaTimezone, getHouseholdTimezone } from '../lib/householdTimezone.js';
 import {
   validateEncryptAndStoreCredential,
   CredentialValidationError,
@@ -51,6 +52,15 @@ const credentialSchema = z.object({
   appPassword: z.string().min(1).max(500),
 });
 
+// Timezone schema — no noEchoHook needed (timezone strings are non-sensitive, T-18-06)
+const timezoneSchema = z.object({
+  timezone: z
+    .string()
+    .min(1)
+    .max(64)
+    .refine(isValidIanaTimezone, { message: 'Invalid IANA timezone identifier' }),
+});
+
 /**
  * noEchoHook: NEVER return result.error from zValidator for credential routes.
  * Zod's error object contains issues[].received which echoes the submitted value
@@ -178,3 +188,78 @@ adminRouter.put('/calendars/:id/shared', async (c) => {
 
   return c.json({ ok: true }, 200);
 });
+
+// ---------------------------------------------------------------------------
+// GET /api/admin/config/timezone
+//
+// Returns the stored household timezone and whether it has been explicitly set
+// (D-01, D-04, D-06). isExplicitlySet: false when no row is in app_config
+// (the response still includes the D-06 fallback as the timezone value).
+// ---------------------------------------------------------------------------
+
+adminRouter.get('/config/timezone', async (c) => {
+  const [row] = await db
+    .select({ value: appConfig.value })
+    .from(appConfig)
+    .where(eq(appConfig.key, 'household_timezone'))
+    .limit(1);
+
+  const isExplicitlySet = row?.value != null;
+  const timezone = isExplicitlySet
+    ? (row.value as string)
+    : await getHouseholdTimezone(db);
+
+  return c.json({ timezone, isExplicitlySet });
+});
+
+// ---------------------------------------------------------------------------
+// PUT /api/admin/config/timezone
+//
+// Validates the IANA timezone string (server-side, T-18-04) and upserts the
+// household_timezone key in app_config (D-01, D-04).
+// Uses onDuplicateKeyUpdate — appConfig.key is the PK so this is a true upsert.
+// No noEchoHook: timezone strings are non-sensitive (T-18-06).
+// ---------------------------------------------------------------------------
+
+adminRouter.put('/config/timezone', zValidator('json', timezoneSchema), async (c) => {
+  const { timezone } = c.req.valid('json');
+
+  await db
+    .insert(appConfig)
+    .values({ key: 'household_timezone', value: timezone })
+    .onDuplicateKeyUpdate({ set: { value: timezone } });
+
+  return c.json({ ok: true }, 200);
+});
+
+// ---------------------------------------------------------------------------
+// POST /api/admin/config/timezone/seed
+//
+// Seeds household_timezone ONLY when currently unset (D-02 first-run / D-03
+// no-overwrite). Used by the Phase 12 setup wizard and the Phase 18 auto-detect
+// flow to store the browser-detected IANA zone without clobbering an admin's
+// explicit choice.
+//
+// Always returns 200 with { ok: true, seeded: <bool> }.
+// Does NOT use onDuplicateKeyUpdate — an explicit SELECT + conditional INSERT
+// ensures the existing value is never overwritten (D-03).
+// ---------------------------------------------------------------------------
+
+adminRouter.post('/config/timezone/seed', zValidator('json', timezoneSchema), async (c) => {
+  const { timezone } = c.req.valid('json');
+
+  const [existing] = await db
+    .select({ value: appConfig.value })
+    .from(appConfig)
+    .where(eq(appConfig.key, 'household_timezone'))
+    .limit(1);
+
+  const alreadySet = existing?.value != null;
+  if (!alreadySet) {
+    await db
+      .insert(appConfig)
+      .values({ key: 'household_timezone', value: timezone });
+  }
+
+  return c.json({ ok: true, seeded: !alreadySet }, 200);
+});