diff --git a/.env.example b/.env.example
index 22c86e7..d2fb4b7 100644
--- a/.env.example
+++ b/.env.example
@@ -14,6 +14,7 @@ AUTH_BOOTSTRAP_PASSWORD=change-me
 
 # Optional: runtime overrides.
 # Leave these commented out to use the application's built-in defaults.
+# TZ=Europe/Amsterdam
 # APP_DEBUG=
 # AUTH_SESSION_COOKIE_NAME=
 # AUTH_SESSION_TTL_HOURS=
diff --git a/app/services/timezone.py b/app/services/timezone.py
index e9e8657..a296a7d 100644
--- a/app/services/timezone.py
+++ b/app/services/timezone.py
@@ -19,8 +19,8 @@ Priority for resolving the local timezone
 -----------------------------------------
 1. ``TZ`` environment variable — ``ZoneInfo(os.environ["TZ"])``.
    Set ``TZ=Europe/Amsterdam`` in the deployment env for correct NL handling.
-2. System local timezone fallback: ``datetime.now().astimezone().tzinfo``.
-   This matches the behaviour callers already relied on implicitly.
+2. The DST-aware ``Europe/Amsterdam`` business timezone.  This keeps local-day
+   calculations deterministic when a deployment does not set ``TZ``.
 """
 
 from __future__ import annotations
@@ -40,7 +40,7 @@ def local_tz() -> "tzinfo":
     Resolution order:
     1. ``TZ`` environment variable (``ZoneInfo(TZ)``).  Set
        ``TZ=Europe/Amsterdam`` in production for correct NL/DST handling.
-    2. System local timezone via ``datetime.now().astimezone().tzinfo``.
+    2. The DST-aware ``Europe/Amsterdam`` business timezone.
 
     **Monkeypatch this function in tests** to get deterministic timezone
     behaviour regardless of CI host configuration::
@@ -51,9 +51,7 @@ def local_tz() -> "tzinfo":
     tz_env = os.environ.get("TZ", "").strip()
     if tz_env:
         return ZoneInfo(tz_env)
-    # System fallback — identical to the .astimezone() pattern already used
-    # in homeassistant_inbound.py and poo.py.
-    return datetime.now().astimezone().tzinfo  # type: ignore[return-value]
+    return ZoneInfo("Europe/Amsterdam")
 
 
 def to_local(dt: datetime) -> datetime:
diff --git a/docker-compose.yml b/docker-compose.yml
index 7019365..c45a9a0 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -6,6 +6,8 @@ services:
     restart: "no"
     init: true
     command: ["python", "-m", "scripts.run_migrations"]
+    environment:
+      TZ: "${TZ:-Europe/Amsterdam}"
     volumes:
       - ./data:/app/data
       - ./.env:/app/.env:ro
@@ -17,6 +19,8 @@ services:
     user: "1000:1000"
     restart: unless-stopped
     init: true
+    environment:
+      TZ: "${TZ:-Europe/Amsterdam}"
     depends_on:
       migration:
         condition: service_completed_successfully
diff --git a/tests/test_deployment.py b/tests/test_deployment.py
index 93e86d5..8b7f707 100644
--- a/tests/test_deployment.py
+++ b/tests/test_deployment.py
@@ -72,6 +72,21 @@ def test_compose_uses_migration_job_before_app() -> None:
     assert dev["services"]["app"]["build"] == "."
 
 
+def test_compose_defaults_business_timezone_and_dev_inherits_it() -> None:
+    """Both production services receive an overridable Amsterdam timezone.
+
+    Compose merges service ``environment`` mappings, and the dev override does
+    not replace either mapping, so the base default applies to base+dev too.
+    """
+    base = _read_yaml("docker-compose.yml")
+    dev = _read_yaml("docker-compose.dev.yml")
+    default_tz = "${TZ:-Europe/Amsterdam}"
+
+    for service_name in ("migration", "app"):
+        assert base["services"][service_name]["environment"]["TZ"] == default_tz
+        assert "TZ" not in dev["services"].get(service_name, {}).get("environment", {})
+
+
 def test_compose_keeps_app_non_root_and_maps_minimal_warmtelink_serial_access() -> None:
     """Base Compose maps the configured device with only pyserial's required access.
 
diff --git a/tests/test_meter_cost_api.py b/tests/test_meter_cost_api.py
index ab461a9..7e55f4b 100644
--- a/tests/test_meter_cost_api.py
+++ b/tests/test_meter_cost_api.py
@@ -5,6 +5,7 @@ from __future__ import annotations
 from datetime import UTC, datetime, timedelta
 from decimal import Decimal
 from unittest.mock import patch
+from zoneinfo import ZoneInfo
 
 import pytest
 from fastapi.testclient import TestClient
@@ -139,7 +140,9 @@ def test_meter_cost_summary_empty_and_recompute_csrf_window_validation(meter_cos
     ).status_code == 422
 
 
-def test_meter_cost_summary_returns_five_fixed_components_and_totals(meter_cost_client) -> None:
+def test_meter_cost_summary_returns_five_fixed_components_and_totals(
+    meter_cost_client, monkeypatch: pytest.MonkeyPatch
+) -> None:
     client, engine = meter_cost_client
     now = datetime.now(UTC)
     values = {
@@ -166,8 +169,10 @@ def test_meter_cost_summary_returns_five_fixed_components_and_totals(meter_cost_
         db.add_all((heating, water))
         db.commit()
     _login(client)
-    with patch("app.api.routes.api.meter_costs.local_now", return_value=datetime(2026, 6, 25, 2, tzinfo=UTC)):
-        response = client.get("/api/energy/meter-costs/summary?scope=thermal&start=2026-06-23T00:00:00Z&end=2026-06-24T00:00:00Z")
+    monkeypatch.setattr("app.services.timezone.local_tz", lambda: ZoneInfo("Europe/Amsterdam"))
+    response = client.get(
+        "/api/energy/meter-costs/summary?scope=thermal&start=2026-06-23T00:00:00Z&end=2026-06-24T00:00:00Z"
+    )
     assert response.status_code == 200
     body = response.json()
     assert body["heating"] == "1.000000000" and body["hot_water_heating"] == "0.8"
diff --git a/tests/test_timezone.py b/tests/test_timezone.py
index 35a34af..4610cf6 100644
--- a/tests/test_timezone.py
+++ b/tests/test_timezone.py
@@ -7,7 +7,7 @@ on any CI host timezone.
 from __future__ import annotations
 
 from datetime import UTC, date, datetime
-from zoneinfo import ZoneInfo
+from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
 
 import pytest
 
@@ -56,6 +56,38 @@ def test_local_tz_env_var_overrides(monkeypatch):
     monkeypatch.delenv("TZ")
 
 
+@pytest.mark.parametrize("value", [None, "", "   "])
+def test_local_tz_defaults_to_dst_aware_amsterdam(monkeypatch, value: str | None):
+    """Unset or blank TZ must use the deterministic Amsterdam business zone."""
+    if value is None:
+        monkeypatch.delenv("TZ", raising=False)
+    else:
+        monkeypatch.setenv("TZ", value)
+
+    tz = tz_mod.local_tz()
+    assert isinstance(tz, ZoneInfo)
+    assert tz.key == "Europe/Amsterdam"
+    winter = datetime(2026, 1, 15, 12, tzinfo=tz)
+    summer = datetime(2026, 7, 15, 12, tzinfo=tz)
+    assert winter.utcoffset().total_seconds() == 3600
+    assert summer.utcoffset().total_seconds() == 7200
+
+
+def test_local_tz_explicit_utc_override(monkeypatch):
+    """A non-empty TZ remains an operator-controlled override."""
+    monkeypatch.setenv("TZ", "UTC")
+    tz = tz_mod.local_tz()
+    assert isinstance(tz, ZoneInfo)
+    assert tz.key == "UTC"
+
+
+def test_local_tz_invalid_explicit_override_fails_loudly(monkeypatch):
+    """A non-empty invalid override must not silently become Amsterdam."""
+    monkeypatch.setenv("TZ", "Invalid/Timezone")
+    with pytest.raises(ZoneInfoNotFoundError):
+        tz_mod.local_tz()
+
+
 # ---------------------------------------------------------------------------
 # to_local() — conversion correctness
 # ---------------------------------------------------------------------------