Backend: UTC nel DB, +00:00 nelle API, ora italiana solo in presentazione (#42)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docker-compose.dev.yml

@@ -14,6 +14,7 @@ services:
       --authentication-policy=mysql_native_password
       --character-set-server=utf8mb4
       --collation-server=utf8mb4_unicode_ci
+      --default-time-zone=+00:00
     healthcheck:
       test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
       interval: 10s

docker-compose.yml

@@ -14,6 +14,7 @@ services:
       --authentication-policy=mysql_native_password
       --character-set-server=utf8mb4
       --collation-server=utf8mb4_unicode_ci
+      --default-time-zone=+00:00
     healthcheck:
       test: ["CMD", "mysqladmin", "ping", "-h", "localhost"]
       interval: 10s

src/backend/api/middleware/api_key.py

@@ -9,7 +9,8 @@ from src.backend.database import get_db
 from src.backend.config import settings
 from src.backend.models.orm.api_key import ApiKey
 from src.backend.models.orm.user import User
-from src.backend.services.auth_service import hash_api_key, utcnow
+from src.backend.services.auth_service import hash_api_key
+from src.backend.utils.tempo import utcnow
 
 _RENEW_INTERVAL = timedelta(minutes=5)
 

src/backend/api/routers/measurements.py

@@ -2,6 +2,8 @@
 import csv
 import io
 from datetime import datetime
+from src.backend.models.api.tipi import UtcNaiveInput
+from src.backend.utils.tempo import as_utc_naive, iso_utc, to_local, utcnow
 
 from fastapi import APIRouter, Depends, HTTPException, Query, status
 from fastapi.responses import StreamingResponse
@@ -232,8 +234,8 @@ async def get_measurements(
     measured_by: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     pass_fail: str | None = Query(None, pattern="^(pass|warning|fail)$"),
     production_run_id: int | None = Query(None, gt=0),
     page: int = Query(1, ge=1),
@@ -290,8 +292,8 @@ async def export_measurements_csv(
     measured_by: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     pass_fail: str | None = Query(None, pattern="^(pass|warning|fail)$"),
     user: User = Depends(get_current_user),
     db: AsyncSession = Depends(get_db),
@@ -372,7 +374,7 @@ async def export_measurements_csv(
             neutralise_csv_cell(m.lot_number or ""),
             neutralise_csv_cell(m.serial_number or ""),
             m.input_method,
-            m.measured_at.isoformat(),
+            iso_utc(m.measured_at),
             m.synced_to_csv,
         ])
 
@@ -382,6 +384,6 @@ async def export_measurements_csv(
         iter([output.getvalue()]),
         media_type="text/csv",
         headers={
-            "Content-Disposition": f"attachment; filename=measurements_{datetime.now().strftime('%Y%m%d_%H%M%S')}.csv"
+            "Content-Disposition": f"attachment; filename=measurements_{to_local(utcnow()).strftime('%Y%m%d_%H%M%S')}.csv"
         },
     )

src/backend/api/routers/reports.py

@@ -1,5 +1,5 @@
 """Reports router — PDF report generation endpoints for Metrologist."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcNaiveInput
 
 from fastapi import APIRouter, Depends, HTTPException, Query, status
 from fastapi.responses import Response
@@ -18,8 +18,8 @@ async def download_spc_report(
     recipe_id: int = Query(...),
     subtask_id: int = Query(...),
     version_id: int | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     operator_id: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),
@@ -52,8 +52,8 @@ async def download_measurement_report(
     recipe_id: int = Query(...),
     subtask_id: int | None = Query(None),
     version_id: int | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     operator_id: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),

src/backend/api/routers/setup.py

@@ -6,6 +6,7 @@ NOT protected by API Key auth (standalone access).
 import hmac
 import random
 from datetime import datetime, timedelta
+from src.backend.utils.tempo import utcnow
 from pathlib import Path
 
 from alembic import command
@@ -555,7 +556,7 @@ async def seed_demo_data(body: PasswordBody):
             tec_user = created_users[2]  # tec1
             lot_numbers = ["LOT-2025-001", "LOT-2025-002", "LOT-2025-003", "LOT-2025-004"]
             serial_numbers = [f"SN-{i:04d}" for i in range(1, 13)]
-            base_date = datetime.now()
+            base_date = utcnow()
 
             all_subtasks = subtasks_1 + subtasks_2
             measurements_created = 0

src/backend/api/routers/statistics.py

@@ -1,5 +1,6 @@
 """Statistics router - SPC endpoints for Metrologist dashboard."""
 from datetime import datetime
+from src.backend.models.api.tipi import UtcNaiveInput
 
 from fastapi import APIRouter, Depends, HTTPException, Query, status
 from sqlalchemy import and_, select
@@ -100,8 +101,8 @@ async def get_summary(
     recipe_id: int = Query(...),
     version_id: int | None = Query(None),
     subtask_id: int | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     operator_id: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),
@@ -122,8 +123,8 @@ async def get_capability(
     recipe_id: int = Query(...),
     subtask_id: int = Query(...),
     version_id: int | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     operator_id: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),
@@ -145,8 +146,8 @@ async def get_control_chart(
     recipe_id: int = Query(...),
     subtask_id: int = Query(...),
     version_id: int | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     operator_id: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),
@@ -172,8 +173,8 @@ async def get_histogram(
     recipe_id: int = Query(...),
     subtask_id: int = Query(...),
     version_id: int | None = Query(None),
-    date_from: datetime | None = Query(None),
+    date_from: UtcNaiveInput = Query(None),
-    date_to: datetime | None = Query(None),
+    date_to: UtcNaiveInput = Query(None),
     operator_id: int | None = Query(None),
     lot_number: str | None = Query(None),
     serial_number: str | None = Query(None),

src/backend/config.py

@@ -28,6 +28,9 @@ class Settings(BaseSettings):
     db_user: str = "tmflow"
     db_password: str = _DEFAULT_DB_PASSWORD
 
+    # Presentation time zone (DB is UTC, #42)
+    display_timezone: str = "Europe/Rome"
+
     # Server
     server_host: str = "0.0.0.0"
     server_port: int = 8000

src/backend/models/api/measurement.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for Measurement operations."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime, UtcNaiveInput
 from typing import Optional
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -54,8 +54,8 @@ class MeasurementResponse(BaseModel):
     # Who let this value stand, when it was out of tolerance. Null on a value in
     # tolerance - and on one that is not and is still waiting for an answer.
     supervisor_id: Optional[int] = None
-    authorised_at: Optional[datetime] = None
+    authorised_at: Optional[UtcDateTime] = None
-    measured_at: datetime
+    measured_at: UtcDateTime
     synced_to_csv: bool
 
 
@@ -97,7 +97,7 @@ class CycleMeasurementResponse(BaseModel):
     subtask_id: int
     value: float
     pass_fail: str
-    measured_at: datetime
+    measured_at: UtcDateTime
 
 
 class CurrentCycleResponse(BaseModel):
@@ -114,8 +114,8 @@ class MeasurementQuery(BaseModel):
     measured_by: Optional[int] = None
     lot_number: Optional[str] = None
     serial_number: Optional[str] = None
-    date_from: Optional[datetime] = None
+    date_from: UtcNaiveInput = None
-    date_to: Optional[datetime] = None
+    date_to: UtcNaiveInput = None
     pass_fail: Optional[str] = Field(None, pattern="^(pass|warning|fail)$")
     page: int = Field(1, ge=1)
     per_page: int = Field(50, ge=1, le=500)

src/backend/models/api/production.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for production runs and their events."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime
 from typing import Literal, Optional
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -21,7 +21,7 @@ class ProductionEventResponse(BaseModel):
     supervisor_id: Optional[int]
     task_id: Optional[int] = None
     note: Optional[str]
-    created_at: datetime
+    created_at: UtcDateTime
 
 
 class ProductionRunResponse(BaseModel):
@@ -36,11 +36,11 @@ class ProductionRunResponse(BaseModel):
     serial_number: Optional[str]
     status: str
     measurement_interval_minutes: Optional[int]
-    next_measurement_at: Optional[datetime]
+    next_measurement_at: Optional[UtcDateTime]
     cycle_count: int
-    started_at: datetime
+    started_at: UtcDateTime
-    paused_at: Optional[datetime]
+    paused_at: Optional[UtcDateTime]
-    closed_at: Optional[datetime]
+    closed_at: Optional[UtcDateTime]
     closed_by: Optional[int]
     statistics_path: Optional[str] = None
 
@@ -49,7 +49,7 @@ class ProductionRunResponse(BaseModel):
     # overdue is a fact the operator must see, not deduce.
     seconds_to_next_measurement: Optional[int] = None
     overdue: bool = False
-    server_time: Optional[datetime] = None
+    server_time: Optional[UtcDateTime] = None
 
     # The measurement tasks of this run's version, in execution order. The client
     # needs them to bring the operator back to the measurement when the interval

src/backend/models/api/recipe.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for Recipe and RecipeVersion operations."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime
 from typing import Optional, TYPE_CHECKING
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -51,7 +51,7 @@ class RecipeVersionResponse(BaseModel):
     version_number: int
     is_current: bool
     created_by: int
-    created_at: datetime
+    created_at: UtcDateTime
     change_notes: Optional[str] = None
     tasks: list["TaskResponse"] = []
 
@@ -72,7 +72,7 @@ class RecipeResponse(BaseModel):
     requires_serial: bool = False
     allow_manual_input: bool = False
     created_by: int
-    created_at: datetime
+    created_at: UtcDateTime
     active: bool
     current_version: Optional[RecipeVersionResponse] = None
 

src/backend/models/api/station.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for Station and StationRecipeAssignment."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime
 from typing import Optional
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -29,7 +29,7 @@ class StationResponse(BaseModel):
     notes: Optional[str]
     active: bool
     created_by: int
-    created_at: datetime
+    created_at: UtcDateTime
 
 
 class StationRecipeAssignmentCreate(BaseModel):
@@ -42,7 +42,7 @@ class StationRecipeAssignmentResponse(BaseModel):
     station_id: int
     recipe_id: int
     assigned_by: int
-    assigned_at: datetime
+    assigned_at: UtcDateTime
 
 
 class RecipeSummary(BaseModel):

src/backend/models/api/statistics.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for SPC statistics responses."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime, UtcNaiveInput
 from typing import Optional
 
 from pydantic import BaseModel, Field
@@ -10,8 +10,8 @@ class StatisticsQuery(BaseModel):
     recipe_id: int
     version_id: Optional[int] = None
     subtask_id: Optional[int] = None
-    date_from: Optional[datetime] = None
+    date_from: UtcNaiveInput = None
-    date_to: Optional[datetime] = None
+    date_to: UtcNaiveInput = None
     operator_id: Optional[int] = None
     lot_number: Optional[str] = None
     serial_number: Optional[str] = None
@@ -20,7 +20,7 @@ class StatisticsQuery(BaseModel):
 class ControlChartData(BaseModel):
     """Data for X-bar/R control chart."""
     values: list[float]
-    timestamps: list[datetime]
+    timestamps: list[UtcDateTime]
     mean: float
     ucl: float  # Upper Control Limit
     lcl: float  # Lower Control Limit
@@ -60,7 +60,7 @@ class CapabilityData(BaseModel):
 
 class TrendData(BaseModel):
     """Temporal trend data for capability indices."""
-    dates: list[datetime]
+    dates: list[UtcDateTime]
     cpk_values: list[Optional[float]]
     ppk_values: list[Optional[float]]
     n_per_period: list[int]

src/backend/models/api/tipi.py

@@ -0,0 +1,14 @@
+"""Shared Pydantic types for datetimes (#42): UTC in the DB, explicit +00:00 on the wire."""
+from datetime import datetime
+from typing import Annotated, Optional
+
+from pydantic import AfterValidator, PlainSerializer
+
+from src.backend.utils.tempo import as_utc_naive, to_utc_aware
+
+UtcDateTime = Annotated[
+    datetime, PlainSerializer(lambda v: to_utc_aware(v).isoformat(), return_type=str)
+]
+
+# Input: aware values are converted to UTC naive, naive is taken as UTC.
+UtcNaiveInput = Annotated[Optional[datetime], AfterValidator(as_utc_naive)]

src/backend/models/api/user.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for User operations."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime
 from typing import Optional
 
 from pydantic import BaseModel, ConfigDict, Field
@@ -48,8 +48,8 @@ class UserResponse(BaseModel):
     language_pref: str
     theme_pref: str
     active: bool
-    created_at: datetime
+    created_at: UtcDateTime
-    last_login: Optional[datetime] = None
+    last_login: Optional[UtcDateTime] = None
 
 
 class UserPasswordChange(BaseModel):

src/backend/models/api/vision.py

@@ -1,5 +1,5 @@
 """Pydantic schemas for the vision execution response."""
-from datetime import datetime
+from src.backend.models.api.tipi import UtcDateTime
 
 from pydantic import BaseModel, ConfigDict
 
@@ -28,4 +28,4 @@ class VisionReferenceImageResponse(BaseModel):
     note: str | None = None
     station_id: int | None = None
     device_code: str | None = None
-    acquired_at: datetime
+    acquired_at: UtcDateTime

src/backend/services/auth_service.py

@@ -1,7 +1,8 @@
 """Authentication service - password hashing, API key management."""
 import hashlib
 import secrets
-from datetime import datetime, timedelta, timezone
+from datetime import datetime, timedelta
+from src.backend.utils.tempo import utcnow
 
 import bcrypt
 from fastapi import HTTPException, status
@@ -78,11 +79,6 @@ async def authorise_supervisor(db: AsyncSession, username: str, password: str) -
     return supervisor
 
 
-def utcnow() -> datetime:
-    """Naive UTC now (the whole DB moves to naive UTC, #42)."""
-    return datetime.now(timezone.utc).replace(tzinfo=None)
-
-
 def hash_api_key(key: str) -> str:
     """SHA-256 hex digest of an API key; this is all the DB ever stores."""
     return hashlib.sha256(key.encode("utf-8")).hexdigest()

src/backend/services/measurement_service.py

@@ -1,6 +1,7 @@
 """Measurement service - pass/fail calculation, data storage."""
 import math
 from datetime import datetime
+from src.backend.utils.tempo import utcnow
 from decimal import Decimal
 from typing import TYPE_CHECKING
 
@@ -343,7 +344,7 @@ async def authorise_measurement(
 ) -> Measurement:
     """Let an out-of-tolerance value stand, on the supervisor's name."""
     measurement.supervisor_id = supervisor.id
-    measurement.authorised_at = datetime.now()
+    measurement.authorised_at = utcnow()
     await db.flush()
     await db.refresh(measurement)
     return measurement

src/backend/services/production_export_service.py

@@ -14,6 +14,7 @@ import csv
 import io
 import logging
 from datetime import datetime
+from src.backend.utils.tempo import iso_utc, to_local, utcnow
 from pathlib import Path
 
 from sqlalchemy import select, update
@@ -169,9 +170,9 @@ async def build_statistics_csv(db: AsyncSession, run: ProductionRun) -> str:
             measurement.input_method,
             "" if measurement.input_duration_ms is None else measurement.input_duration_ms,
             measurement.measured_by,
-            measurement.measured_at.isoformat() if measurement.measured_at else "",
+            iso_utc(measurement.measured_at),
             measurement.supervisor_id or "",
-            measurement.authorised_at.isoformat() if measurement.authorised_at else "",
+            iso_utc(measurement.authorised_at),
         ])
 
     return output.getvalue()
@@ -191,7 +192,7 @@ async def export_run_statistics(db: AsyncSession, run: ProductionRun) -> str | N
     directory = Path(settings.upload_path) / EXPORT_SUBDIR
     directory.mkdir(parents=True, exist_ok=True)
 
-    stamp = (run.closed_at or datetime.now()).strftime("%Y%m%d_%H%M%S")
+    stamp = to_local(run.closed_at or utcnow()).strftime("%Y%m%d_%H%M%S")
     filename = f"production_{run.id}_{stamp}.csv"
     # A large run's file can take a moment to write: off the event loop, so
     # closing one run does not stall every other request being served.

src/backend/services/production_service.py

@@ -14,6 +14,7 @@ the countdown would be off by the UTC offset. The number is the contract; the
 timestamp is only there for the audit trail.
 """
 from datetime import datetime, timedelta
+from src.backend.utils.tempo import utcnow
 from typing import Optional
 
 from fastapi import HTTPException, status
@@ -34,10 +35,10 @@ from src.backend.services import (
 def _now() -> datetime:
     """Single source of time for runs.
 
-    Naive local time, matching what the database writes for ``func.now()`` defaults,
+    Naive UTC, matching what the database writes for ``func.now()`` defaults,
     so a run's started_at and its deadline are comparable.
     """
-    return datetime.now()
+    return utcnow()
 
 
 # ---------------------------------------------------------------------------

src/backend/services/report_service.py

@@ -2,6 +2,7 @@
 import asyncio
 import base64
 from datetime import datetime
+from src.backend.utils.tempo import to_local, utcnow
 from pathlib import Path
 
 import plotly.graph_objects as go
@@ -342,7 +343,7 @@ async def generate_spc_report(
         control_chart_svg=control_chart_svg,
         histogram_svg=histogram_svg,
         filters_desc=filters_desc,
-        generated_at=datetime.now(),
+        generated_at=to_local(utcnow()),
         n_measurements=len(measurements),
     )
 
@@ -395,7 +396,7 @@ async def generate_measurement_report(
             "value": f"{float(m.value):.4f}",
             "unit": st_info.get("unit", "mm"),
             "pass_fail": m.pass_fail,
-            "measured_at": m.measured_at.strftime("%Y-%m-%d %H:%M"),
+            "measured_at": to_local(m.measured_at).strftime("%Y-%m-%d %H:%M"),
             "operator": operator_map.get(m.measured_by, f"ID {m.measured_by}"),
             "lot_number": m.lot_number or "—",
             "serial_number": m.serial_number or "—",
@@ -430,7 +431,7 @@ async def generate_measurement_report(
         rows=rows,
         summary=summary,
         filters_desc=filters_desc,
-        generated_at=datetime.now(),
+        generated_at=to_local(utcnow()),
         n_measurements=len(measurements),
     )
 

src/backend/tests/test_tempo.py

@@ -0,0 +1,108 @@
+"""UTC in the DB, explicit +00:00 on the wire, local time only on presentation (#42)."""
+from datetime import datetime, timezone
+from unittest.mock import patch
+
+from httpx import AsyncClient
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from src.backend.models.orm.user import User
+from src.backend.services import report_service
+from src.backend.tests.conftest import auth_headers, create_test_recipe
+from src.backend.utils.tempo import as_utc_naive, to_local, utcnow
+
+
+def test_utcnow_naive_and_close_to_utc():
+    now = utcnow()
+    assert now.tzinfo is None
+    delta = datetime.now(timezone.utc).replace(tzinfo=None) - now
+    assert abs(delta.total_seconds()) < 5
+
+
+def test_as_utc_naive():
+    parsed = datetime.fromisoformat("2026-10-06T10:00:00+02:00")
+    assert as_utc_naive(parsed) == datetime(2026, 10, 6, 8, 0)
+    assert as_utc_naive(parsed).tzinfo is None
+    naive = datetime(2026, 10, 6, 8, 0)
+    assert as_utc_naive(naive) is naive
+    assert as_utc_naive(None) is None
+
+
+def test_to_local_is_rome():
+    assert to_local(datetime(2026, 10, 6, 8, 0)).strftime("%H:%M %z") == "10:00 +0200"
+    assert to_local(datetime(2026, 1, 6, 8, 0)).strftime("%H:%M %z") == "09:00 +0100"
+
+
+async def _make_measurement(client, user, recipe_id) -> None:
+    tasks = (await client.get(f"/api/recipes/{recipe_id}/tasks", headers=auth_headers(user))).json()
+    await client.post(
+        "/api/measurements/",
+        headers=auth_headers(user),
+        json={
+            "subtask_id": tasks[0]["subtasks"][0]["id"],
+            "version_id": tasks[0]["version_id"],
+            "value": 10.0,
+        },
+    )
+
+
+async def test_api_datetimes_carry_utc_offset_and_filter_equivalence(
+    client: AsyncClient,
+    measurement_tec_user: User,
+    metrologist_user: User,
+    db_session: AsyncSession,
+):
+    recipe = await create_test_recipe(db_session, measurement_tec_user.id)
+    await _make_measurement(client, measurement_tec_user, recipe.id)
+
+    resp = await client.get("/api/measurements/", headers=auth_headers(metrologist_user))
+    item = resp.json()["items"][0]
+    assert item["measured_at"].endswith("+00:00")
+
+    # Same instant written with +02:00 and as UTC: identical results.
+    future_utc = "2099-01-01T08:00:00+00:00"
+    future_rome = "2099-01-01T10:00:00+02:00"
+    past_utc = "2000-01-01T08:00:00+00:00"
+    past_rome = "2000-01-01T10:00:00+02:00"
+    for frm, to in ((past_utc, future_utc), (past_rome, future_rome)):
+        r = await client.get(
+            "/api/measurements/",
+            headers=auth_headers(metrologist_user),
+            params={"date_from": frm, "date_to": to},
+        )
+        assert r.status_code == 200
+        assert r.json()["total"] == 1
+
+    # Boundary: measurement is "now" in UTC; a date_from 1h ahead in UTC,
+    # written as +02:00 (now+3h local wall clock), must exclude it.
+    ahead = (utcnow().replace(microsecond=0)).replace(tzinfo=timezone.utc)
+    from datetime import timedelta
+    ahead_utc = (ahead + timedelta(hours=1)).isoformat()
+    ahead_rome = (ahead + timedelta(hours=1)).astimezone(
+        timezone(timedelta(hours=2))
+    ).isoformat()
+    totals = []
+    for frm in (ahead_utc, ahead_rome):
+        r = await client.get(
+            "/api/measurements/",
+            headers=auth_headers(metrologist_user),
+            params={"date_from": frm},
+        )
+        totals.append(r.json()["total"])
+    assert totals == [0, 0]
+
+
+async def test_report_generated_at_is_local_time(
+    db_session: AsyncSession, metrologist_user: User
+):
+    recipe = await create_test_recipe(db_session, metrologist_user.id)
+    captured: dict[str, str] = {}
+
+    def fake_render(html: str) -> bytes:
+        captured["html"] = html
+        return b"%PDF"
+
+    with patch.object(report_service, "utcnow", return_value=datetime(2026, 10, 6, 8, 0)), \
+            patch.object(report_service, "_render_pdf", side_effect=fake_render):
+        await report_service.generate_measurement_report(db_session, recipe.id)
+
+    assert "Generated 2026-10-06 10:00" in captured["html"]

src/backend/utils/tempo.py

@@ -0,0 +1,34 @@
+"""Time helpers: the DB holds naive UTC, presentation converts to display_timezone (#42)."""
+from datetime import datetime, timezone
+from zoneinfo import ZoneInfo
+
+from src.backend.config import settings
+
+
+def utcnow() -> datetime:
+    """Naive UTC now (the whole DB moves to naive UTC, #42)."""
+    return datetime.now(timezone.utc).replace(tzinfo=None)
+
+
+def as_utc_naive(dt: datetime | None) -> datetime | None:
+    """Aware -> UTC naive; naive is taken as UTC already; None stays None."""
+    if dt is None or dt.tzinfo is None:
+        return dt
+    return dt.astimezone(timezone.utc).replace(tzinfo=None)
+
+
+def to_utc_aware(dt: datetime) -> datetime:
+    """Naive (UTC) or aware datetime -> aware UTC."""
+    if dt.tzinfo is None:
+        return dt.replace(tzinfo=timezone.utc)
+    return dt.astimezone(timezone.utc)
+
+
+def iso_utc(dt: datetime | None) -> str:
+    """ISO 8601 with explicit +00:00; "" for None (CSV cells)."""
+    return to_utc_aware(dt).isoformat() if dt else ""
+
+
+def to_local(dt: datetime) -> datetime:
+    """Naive UTC (or aware) datetime -> aware datetime in settings.display_timezone."""
+    return to_utc_aware(dt).astimezone(ZoneInfo(settings.display_timezone))