diff --git a/api/routes_panel.py b/api/routes_panel.py
index 200c431..872dba8 100644
--- a/api/routes_panel.py
+++ b/api/routes_panel.py
@@ -9,8 +9,9 @@ this router inherits the check).
 The snapshot route is a thin adapter over the bundled core: it performs
 NO business logic.  It reads the live database off ``app.state.db``,
 resolves ONE timezone-aware ``now`` (the host clock re-expressed in the
-stored household ``timezone`` setting — the API host may run UTC while
-the household does not), calls the pure
+household ``timezone`` — the API host may run UTC while the household
+does not; until the admin chooses a zone it is the Home Assistant zone
+the integration reports in :data:`PANEL_TIMEZONE_HEADER`), calls the pure
 :func:`nestquest_core.snapshot.build_snapshot`, shapes each child's
 instances through :func:`nestquest_core.snapshot.instance_payload` with
 ``include_missed=False`` (D-009: missed instances are OMITTED from the
@@ -53,6 +54,7 @@ from __future__ import annotations
 
 import datetime
 import importlib
+import logging
 
 from fastapi import APIRouter, Depends, HTTPException, Request
 from fastapi.responses import StreamingResponse
@@ -62,6 +64,7 @@ from api.database import DatabaseState
 from api.dependencies import require_panel_token
 from api.nestquest_core import (
     core_completion,
+    core_const,
     core_events,
     core_settings_store,
     core_snapshot,
@@ -69,6 +72,12 @@ from api.nestquest_core import (
 from api import transitions as api_transitions
 from api.sse import transition_event_stream
 
+LOGGER = logging.getLogger(__name__)
+
+#: The request header the integration reports Home Assistant's
+#: configured IANA ``time_zone`` in.
+PANEL_TIMEZONE_HEADER = core_const.PANEL_TIMEZONE_HEADER
+
 #: The instance DAO, imported on the app's core copy for the
 #: child-mismatch guard on the complete route.
 _dao_instances = importlib.import_module("nestquest_core.dao_instances")
@@ -179,16 +188,35 @@ def _local_now() -> datetime.datetime:
 
 
 async def _household_clock(
-    database: object,
+    database: object, request: Request
 ) -> tuple[object, datetime.datetime]:
-    """Return the stored settings and ONE household-local clock read.
+    """Return the settings and ONE household-local clock read.
 
     The API host may run UTC while the household does not, so the one
-    :func:`_local_now` read is re-expressed in the stored settings'
-    ``timezone`` (:meth:`NestQuestSettings.household_now`); an unset
-    zone keeps the host's local time.
+    :func:`_local_now` read is re-expressed in the settings' ``timezone``
+    (:meth:`NestQuestSettings.household_now`).  The integration reports
+    Home Assistant's ``time_zone`` in :data:`PANEL_TIMEZONE_HEADER`;
+    until the admin chooses a zone it is adopted and persisted
+    (:func:`nestquest_core.settings_store.adopt_reported_timezone`), so
+    the scheduler and the admin plane follow it too.  An invalid
+    reported zone is logged and ignored — the panel keeps serving on
+    the stored settings.
     """
-    settings = await core_settings_store.load_settings(database)
+    reported = request.headers.get(PANEL_TIMEZONE_HEADER, "").strip()
+    if reported:
+        try:
+            settings = await core_settings_store.adopt_reported_timezone(
+                database, reported
+            )
+        except ValueError:
+            LOGGER.warning(
+                "Ignoring invalid %s header value %r",
+                PANEL_TIMEZONE_HEADER,
+                reported,
+            )
+            settings = await core_settings_store.load_settings(database)
+    else:
+        settings = await core_settings_store.load_settings(database)
     return settings, settings.household_now(_local_now())
 
 
@@ -210,7 +238,7 @@ async def panel_snapshot(request: Request) -> PanelSnapshotResponse:
     :class:`PanelSnapshotResponse`.
     """
     state: DatabaseState = request.app.state.db
-    settings, now = await _household_clock(state.database)
+    settings, now = await _household_clock(state.database, request)
     snapshot = await core_snapshot.build_snapshot(
         state.database, settings, now
     )
@@ -297,7 +325,7 @@ async def panel_complete_instance(
             detail="Quest instance not found for this child",
         )
 
-    _settings, now = await _household_clock(database)
+    _settings, now = await _household_clock(database, request)
     try:
         result = await core_completion.complete_instance(
             database,
diff --git a/custom_components/nestquest/api_client.py b/custom_components/nestquest/api_client.py
index 7dbf157..b258be5 100644
--- a/custom_components/nestquest/api_client.py
+++ b/custom_components/nestquest/api_client.py
@@ -6,6 +6,9 @@ speaks the API's panel plane — ``GET /api/v1/panel/snapshot`` (today's
 household snapshot) and ``POST /api/v1/panel/instances/{id}/complete``
 (completion as the tapped child profile) — authenticated with the
 static panel service token as a Bearer credential on every request.
+Every request also reports Home Assistant's configured ``time_zone`` in
+the ``X-NestQuest-Timezone`` header, so the API (which may run UTC)
+reads the household's clock until the admin chooses a zone there.
 
 Transport-injected by construction: the constructor takes the base URL,
 the panel token, and the HTTP session to use.  The session speaks the
@@ -54,6 +57,7 @@ from .const import (
     CONF_PANEL_TOKEN,
     DEFAULT_API_BASE_URL,
     DEFAULT_PANEL_TOKEN,
+    PANEL_TIMEZONE_HEADER,
 )
 
 if TYPE_CHECKING:
@@ -156,7 +160,8 @@ def client_from_entry(hass: Any, entry: Any) -> NestQuestApiClient:
     """Build the production client for a config entry.
 
     Reads the configured base URL and panel token off the entry
-    (:func:`resolve_api_config`) and hands the client Home Assistant's
+    (:func:`resolve_api_config`), reports ``hass.config.time_zone`` as
+    the household zone, and hands the client Home Assistant's
     SHARED aiohttp session (``homeassistant.helpers.aiohttp_client
     .async_get_clientsession``) so panel calls ride HA's one connection
     pool.  The helper import is deferred into this function on purpose:
@@ -168,7 +173,10 @@ def client_from_entry(hass: Any, entry: Any) -> NestQuestApiClient:
 
     base_url, panel_token = resolve_api_config(entry)
     return NestQuestApiClient(
-        base_url, panel_token, async_get_clientsession(hass)
+        base_url,
+        panel_token,
+        async_get_clientsession(hass),
+        household_timezone=hass.config.time_zone,
     )
 
 
@@ -189,9 +197,14 @@ class NestQuestApiClient:
         session: aiohttp.ClientSession,
         *,
         timeout_seconds: float = DEFAULT_TIMEOUT_SECONDS,
+        household_timezone: str | None = None,
     ) -> None:
         """Store the connection settings; the session is injected.
 
+        ``household_timezone`` is Home Assistant's configured IANA
+        ``time_zone``; when given it rides every request in the
+        ``X-NestQuest-Timezone`` header.
+
         ``session`` is an aiohttp ``ClientSession`` in production and a
         stub transport in tests.  ``base_url`` must be a full http(s)
         URL and ``panel_token`` must be non-empty — a client built
@@ -216,6 +229,14 @@ class NestQuestApiClient:
         self._panel_token = panel_token
         self._session = session
         self._timeout_seconds = float(timeout_seconds)
+        self._household_timezone = household_timezone
+
+    def _headers(self) -> dict[str, str]:
+        """The Bearer credential plus the reported household zone."""
+        headers = {"Authorization": f"Bearer {self._panel_token}"}
+        if self._household_timezone:
+            headers[PANEL_TIMEZONE_HEADER] = self._household_timezone
+        return headers
 
     async def get_snapshot(self) -> dict[str, Any]:
         """Fetch and parse today's household snapshot.
@@ -275,7 +296,7 @@ class NestQuestApiClient:
         cleanly and closes the underlying response in every path.
         """
         url = f"{self._base_url}{EVENTS_PATH}"
-        headers = {"Authorization": f"Bearer {self._panel_token}"}
+        headers = self._headers()
         # Opening the response gets ONE request timeout, exactly like
         # ``_request_json``; the read loop below runs WITHOUT one — a
         # body-wide timeout would kill a healthy long-lived stream.
@@ -379,7 +400,7 @@ class NestQuestApiClient:
         raises typed too, carrying the status it arrived with.
         """
         url = f"{self._base_url}{path}"
-        headers = {"Authorization": f"Bearer {self._panel_token}"}
+        headers = self._headers()
         try:
             async with asyncio.timeout(self._timeout_seconds):
                 async with self._session.request(
diff --git a/custom_components/nestquest/coordinator.py b/custom_components/nestquest/coordinator.py
index 22cbb24..88bebf6 100644
--- a/custom_components/nestquest/coordinator.py
+++ b/custom_components/nestquest/coordinator.py
@@ -372,7 +372,10 @@ def coordinator_client_from_entry(
         return _UnconfiguredApiClient()
     if session_factory is not None:
         return NestQuestApiClient(
-            base_url, panel_token, session_factory(hass)
+            base_url,
+            panel_token,
+            session_factory(hass),
+            household_timezone=hass.config.time_zone,
         )
     from custom_components.nestquest.api_client import client_from_entry
 
diff --git a/custom_components/nestquest/core/const.py b/custom_components/nestquest/core/const.py
index e1b1625..96f51af 100644
--- a/custom_components/nestquest/core/const.py
+++ b/custom_components/nestquest/core/const.py
@@ -113,18 +113,25 @@ CONF_END_OF_DAY_REPORT_ENABLED = "end_of_day_report_enabled"
 CONF_CELEBRATION_ENABLED = "celebration_enabled"
 DEFAULT_AUTOMATION_ENABLED = True
 # The household's IANA time zone (e.g. ``America/New_York``) the API
-# service reads its clock in.  The empty string keeps the historical
-# behaviour — the API host's own local time — so an existing install is
-# unchanged until the owner sets it.  The integration itself always uses
-# Home Assistant's configured ``time_zone`` and ignores this field.
+# service reads its clock in.  The empty string is the API host's own
+# local time.  While the admin has not chosen a zone
+# (``timezone_configured`` is ``False``) the API adopts the zone the
+# integration reports from Home Assistant's configured ``time_zone``
+# (:data:`PANEL_TIMEZONE_HEADER`), so the default is the household's
+# zone without any manual setup.  The integration itself always uses
+# Home Assistant's ``time_zone`` and ignores this field.
 CONF_TIMEZONE = "timezone"
 DEFAULT_TIMEZONE = ""
 # Whether the admin has made an explicit ``timezone`` choice.  ``False``
-# (a fresh install) lets the admin UI auto-set the browser's zone once;
-# ``True`` pins the stored value — including an explicitly-chosen empty
-# ``timezone`` (the API host's local time) — so it is never auto-set again.
+# (a fresh install) lets the API follow Home Assistant's reported zone
+# and the admin UI auto-set the browser's zone once while none is
+# stored; ``True`` pins the stored value — including an explicitly-chosen
+# empty ``timezone`` (the API host's local time) — so neither changes it.
 CONF_TIMEZONE_CONFIGURED = "timezone_configured"
 DEFAULT_TIMEZONE_CONFIGURED = False
+# The panel-plane request header carrying Home Assistant's configured
+# IANA ``time_zone`` from the integration to the API service.
+PANEL_TIMEZONE_HEADER = "X-NestQuest-Timezone"
 
 # Panel-plane API client (Feature 18): the integration reaches the
 # NestQuest API service (Feature 16) over HTTP instead of the database.
diff --git a/custom_components/nestquest/core/settings.py b/custom_components/nestquest/core/settings.py
index 7ce0e1b..878a668 100644
--- a/custom_components/nestquest/core/settings.py
+++ b/custom_components/nestquest/core/settings.py
@@ -327,12 +327,14 @@ class NestQuestSettings:
     def household_now(self, instant: datetime.datetime) -> datetime.datetime:
         """Return the aware ``instant`` expressed in the household's zone.
 
-        The API service has no Home Assistant ``time_zone``, so its one
+        The API service does not run inside Home Assistant, so its one
         clock read (an aware host-local ``now``) is converted here: with
-        :attr:`timezone` set the SAME instant is re-expressed in that
-        zone, so ``.date()`` and ``.time()`` are the household's wall
-        clock even when the host runs UTC.  With :attr:`timezone` empty
-        the instant is returned unchanged (the host's local time).
+        :attr:`timezone` set — the admin's choice, or else the zone the
+        integration reports from Home Assistant — the SAME instant is
+        re-expressed in that zone, so ``.date()`` and ``.time()`` are the
+        household's wall clock even when the host runs UTC.  With
+        :attr:`timezone` empty the instant is returned unchanged (the
+        host's local time).
         """
         if not self.timezone:
             return instant
diff --git a/custom_components/nestquest/core/settings_store.py b/custom_components/nestquest/core/settings_store.py
index b19800e..586dbbc 100644
--- a/custom_components/nestquest/core/settings_store.py
+++ b/custom_components/nestquest/core/settings_store.py
@@ -36,7 +36,8 @@ after the first update:
   this field existed simply lacks it and resolves to ``""``,
 - ``timezone_configured`` — a real boolean: ``True`` once the admin
   explicitly chose ``timezone`` (including the empty host-local value),
-  so the admin UI never auto-sets it again.  A document stored before
+  so neither Home Assistant's reported zone
+  (:func:`adopt_reported_timezone`) nor the admin UI changes it again.  A document stored before
   this field existed lacks it and resolves to ``False``.
 
 Only values whose core validators ACCEPTED are ever written, so a
@@ -200,18 +201,49 @@ async def update_settings(
         )
     async with _settings_lock(database):
         current = await load_settings(database)
-        document: dict[str, Any] = {
-            field: getattr(current, field) for field in _FIELD_NAMES
-        }
-        for name, raw_value in changes.items():
-            default, validator = _RESOLUTIONS[name]
-            try:
-                document[name] = validator(
-                    default if raw_value is None else raw_value
-                )
-            except ValueError as error:
-                raise ValueError(_named(name, error)) from error
-        await MetaStateDao(database).set(
-            SETTINGS_META_KEY, json.dumps(document, sort_keys=True)
-        )
+        return await _write_merged(database, current, changes)
+
+
+async def adopt_reported_timezone(
+    database: NestQuestDatabase, zone: str
+) -> NestQuestSettings:
+    """Adopt Home Assistant's reported ``zone`` unless the admin chose one.
+
+    While ``timezone_configured`` is ``False`` the stored ``timezone``
+    follows the zone the integration reports, so the API's clock is the
+    household's without any manual setup; ``timezone_configured`` stays
+    ``False`` (the value is automatic, not an admin choice).  Once the
+    admin has chosen — including an explicitly-chosen empty zone — the
+    stored settings are returned untouched.  The check and the write
+    share the settings lock, so a concurrent admin choice is never
+    overwritten.  An invalid ``zone`` raises a ``ValueError`` naming
+    ``timezone`` and writes nothing.
+    """
+    async with _settings_lock(database):
+        current = await load_settings(database)
+        if current.timezone_configured or current.timezone == zone:
+            return current
+        return await _write_merged(database, current, {"timezone": zone})
+
+
+async def _write_merged(
+    database: NestQuestDatabase,
+    current: NestQuestSettings,
+    changes: Mapping[str, Any],
+) -> NestQuestSettings:
+    """Validate ``changes``, merge onto ``current``, persist; lock held."""
+    document: dict[str, Any] = {
+        field: getattr(current, field) for field in _FIELD_NAMES
+    }
+    for name, raw_value in changes.items():
+        default, validator = _RESOLUTIONS[name]
+        try:
+            document[name] = validator(
+                default if raw_value is None else raw_value
+            )
+        except ValueError as error:
+            raise ValueError(_named(name, error)) from error
+    await MetaStateDao(database).set(
+        SETTINGS_META_KEY, json.dumps(document, sort_keys=True)
+    )
     return NestQuestSettings(**document)
diff --git a/tests/test_api_client.py b/tests/test_api_client.py
index 4e9d5a9..9dcc893 100644
--- a/tests/test_api_client.py
+++ b/tests/test_api_client.py
@@ -196,6 +196,27 @@ async def test_complete_instance_posts_body_and_headers() -> None:
     assert call["headers"]["Authorization"] == f"Bearer {TOKEN}"
 
 
+async def test_household_timezone_rides_every_request_header() -> None:
+    """Home Assistant's zone is reported so the API reads the household
+    clock (task 6351b36d); it is sent only when the client has one."""
+    transport = StubTransport(
+        StubResponse(200, json.dumps(SNAPSHOT_PAYLOAD)),
+        StubResponse(200, '{"status": "done"}'),
+    )
+    client = NestQuestApiClient(
+        BASE_URL, TOKEN, transport, household_timezone="America/New_York"
+    )
+
+    await client.get_snapshot()
+    await client.complete_instance(42, 3)
+
+    for call in transport.calls:
+        assert call["headers"] == {
+            "Authorization": f"Bearer {TOKEN}",
+            "X-NestQuest-Timezone": "America/New_York",
+        }
+
+
 async def test_base_url_trailing_slash_is_normalized() -> None:
     """A base URL with a trailing slash must not double the path slash."""
     transport = StubTransport(StubResponse(200, "{}"))
@@ -392,11 +413,20 @@ def test_client_from_entry_reads_config_and_shared_session(monkeypatch) -> None:
         }
     )
 
-    client = api_client.client_from_entry(object(), entry)
+    hass = types.SimpleNamespace(
+        config=types.SimpleNamespace(time_zone="America/New_York")
+    )
+
+    client = api_client.client_from_entry(hass, entry)
 
     assert client._session is shared_session
     assert client._base_url == "http://panel.lan:8443"
     assert client._panel_token == "entry-token"
+    assert client._household_timezone == "America/New_York"
+
+
+#: A minimal hass stand-in carrying the configured time zone.
+_HASS_UTC = types.SimpleNamespace(config=types.SimpleNamespace(time_zone="UTC"))
 
 
 def test_client_from_entry_unconfigured_token_raises_at_construction(
@@ -419,12 +449,12 @@ def test_client_from_entry_unconfigured_token_raises_at_construction(
     assert resolve_api_config(entry)[1] == DEFAULT_PANEL_TOKEN == ""
 
     with pytest.raises(ValueError, match="panel_token must be a non-empty"):
-        api_client.client_from_entry(object(), entry)
+        api_client.client_from_entry(_HASS_UTC, entry)
 
     # Same for a whitespace-only stored token: whitespace is not a token.
     entry_ws = make_config_entry(options={CONF_PANEL_TOKEN: "   "})
     with pytest.raises(ValueError, match="panel_token must be a non-empty"):
-        api_client.client_from_entry(object(), entry_ws)
+        api_client.client_from_entry(_HASS_UTC, entry_ws)
 
 
 async def test_client_logs_nothing() -> None:
diff --git a/tests/test_api_household_clock.py b/tests/test_api_household_clock.py
index 613263d..80e5404 100644
--- a/tests/test_api_household_clock.py
+++ b/tests/test_api_household_clock.py
@@ -326,3 +326,118 @@ def test_delay_seconds_is_true_elapsed_time_across_dst(
     now: datetime.datetime, target: datetime.time, expected: float
 ) -> None:
     assert _delay_seconds(target, now) == expected
+
+
+# --- the Home Assistant-reported zone (task 6351b36d) ------------------------
+#
+# The deployed household never set ``timezone``, so the UTC API host marked
+# morning quests overdue.  The integration reports Home Assistant's
+# ``time_zone`` on every panel request; while the admin has not chosen a
+# zone the API adopts it (persisted, so the scheduler and the admin plane
+# follow too), and an explicit admin choice still wins.
+
+
+def _reporting_panel_headers(zone: str) -> dict[str, str]:
+    return {**_panel_headers(), "X-NestQuest-Timezone": zone}
+
+
+async def _panel_pack_bag(
+    household: SimpleNamespace, zone: str
+) -> tuple[str, dict]:
+    response = await household.client.get(
+        "/api/v1/panel/snapshot", headers=_reporting_panel_headers(zone)
+    )
+    assert response.status_code == 200
+    body = response.json()
+    ada = next(
+        child
+        for child in body["children"]
+        if child["child_id"] == household.seed.ada.id
+    )
+    (instance,) = ada["instances"]
+    return body["today_iso"], instance
+
+
+async def _stored_settings(household: SimpleNamespace) -> dict:
+    response = await household.client.get(
+        "/api/v1/admin/settings", headers=_admin_headers()
+    )
+    assert response.status_code == 200
+    return response.json()
+
+
+async def test_unset_zone_adopts_reported_zone_for_overdue(
+    household: SimpleNamespace,
+) -> None:
+    """09:59:59 EDT on a UTC host, zone never set: NOT overdue."""
+    household.clock["now"] = _utc(13, 59, 59)
+    today_iso, instance = await _panel_pack_bag(household, HOUSEHOLD_ZONE)
+    assert today_iso == TODAY.isoformat()
+    assert instance["overdue"] is False
+    # Persisted as the automatic (not admin-chosen) household zone, so
+    # the admin plane and the missed-sweep scheduler read it too.
+    stored = await _stored_settings(household)
+    assert stored["timezone"] == HOUSEHOLD_ZONE
+    assert stored["timezone_configured"] is False
+    _today, admin_instance = await _pack_bag(household, "admin")
+    assert admin_instance["overdue"] is False
+
+
+async def test_unset_zone_adopts_reported_zone_for_today(
+    household: SimpleNamespace,
+) -> None:
+    """23:30 EDT is tomorrow in UTC; the household day is still today."""
+    household.clock["now"] = _utc(3, 30, day=27)
+    today_iso, _instance = await _panel_pack_bag(household, HOUSEHOLD_ZONE)
+    assert today_iso == TODAY.isoformat()
+
+
+async def test_reported_zone_completion_is_on_time(
+    household: SimpleNamespace,
+) -> None:
+    """A 09:30 EDT tap with the zone never set is on time."""
+    household.clock["now"] = _utc(13, 30)
+    seed = household.seed
+    response = await household.client.post(
+        f"/api/v1/panel/instances/{seed.pack_bag_instance.id}/complete",
+        headers=_reporting_panel_headers(HOUSEHOLD_ZONE),
+        json={"actor_child_id": seed.ada.id},
+    )
+    assert response.status_code == 200
+    _today_iso, instance = await _pack_bag(household, "admin")
+    assert instance["on_time"] is True
+
+
+async def test_reported_zone_follows_home_assistant_changes(
+    household: SimpleNamespace,
+) -> None:
+    """An automatically adopted zone tracks a later Home Assistant change."""
+    await _panel_pack_bag(household, "America/Chicago")
+    await _panel_pack_bag(household, HOUSEHOLD_ZONE)
+    assert (await _stored_settings(household))["timezone"] == HOUSEHOLD_ZONE
+
+
+@pytest.mark.parametrize("chosen", ["America/Los_Angeles", ""])
+async def test_admin_chosen_zone_wins_over_reported_zone(
+    household: SimpleNamespace, chosen: str
+) -> None:
+    response = await household.client.patch(
+        "/api/v1/admin/settings",
+        json={"timezone": chosen, "timezone_configured": True},
+        headers=_admin_headers(),
+    )
+    assert response.status_code == 200
+    await _panel_pack_bag(household, HOUSEHOLD_ZONE)
+    stored = await _stored_settings(household)
+    assert stored["timezone"] == chosen
+    assert stored["timezone_configured"] is True
+
+
+async def test_invalid_reported_zone_is_ignored(
+    household: SimpleNamespace,
+) -> None:
+    household.clock["now"] = _utc(13, 59, 59)
+    _today_iso, instance = await _panel_pack_bag(household, "Mars/Olympus_Mons")
+    # Host-local (UTC) behaviour, unchanged store.
+    assert instance["overdue"] is True
+    assert (await _stored_settings(household))["timezone"] == ""
diff --git a/tests/test_coordinator.py b/tests/test_coordinator.py
index e09a8ab..9d9ee72 100644
--- a/tests/test_coordinator.py
+++ b/tests/test_coordinator.py
@@ -686,6 +686,8 @@ async def test_coordinator_configured_token_builds_real_client(
         hass, entry, session_factory=_session_factory
     )
     assert isinstance(real_client, NestQuestApiClient)
+    # Home Assistant's zone is reported to the API (task 6351b36d).
+    assert real_client._household_timezone == hass.config.time_zone
 
     set_coordinator_client(entry, real_client)
     assert await async_setup_entry(hass, entry) is True