Fixes

The `datetime` variable is timezone-naive. Using
`dt_util.as_utc(datetime)` incorrectly assigns the local timezone
instead of UTC, leading to incorrect conversions. To fix this,
directly assign the UTC timezone to the `datetime` variable before
conversion, ensuring `dt_util.as_utc()` does not overwrite it with the
local timezone.

homeassistant/components/open_meteo/weather.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+from datetime import datetime, time
+
 from open_meteo import Forecast as OpenMeteoForecast
 
 from homeassistant.components.weather import (
@@ -107,8 +109,9 @@ class OpenMeteoWeatherEntity(
 
         daily = self.coordinator.data.daily
         for index, date in enumerate(self.coordinator.data.daily.time):
+            _datetime = datetime.combine(date=date, time=time(0), tzinfo=dt_util.UTC)
             forecast = Forecast(
-                datetime=date.isoformat(),
+                datetime=_datetime.isoformat(),
             )
 
             if daily.weathercode is not None:
@@ -155,12 +158,14 @@ class OpenMeteoWeatherEntity(
         today = dt_util.utcnow()
 
         hourly = self.coordinator.data.hourly
-        for index, datetime in enumerate(self.coordinator.data.hourly.time):
-            if dt_util.as_utc(datetime) < today:
+        for index, _datetime in enumerate(self.coordinator.data.hourly.time):
+            if _datetime.tzinfo is None:
+                _datetime = _datetime.replace(tzinfo=dt_util.UTC)
+            if _datetime < today:
                 continue
 
             forecast = Forecast(
-                datetime=datetime.isoformat(),
+                datetime=_datetime.isoformat(),
             )
 
             if hourly.weather_code is not None:

tests/components/open_meteo/test_weather.py

@@ -0,0 +1,46 @@
+"""Test for the open meteo weather entity."""
+
+from unittest.mock import AsyncMock
+
+import pytest
+from syrupy.assertion import SnapshotAssertion
+
+from homeassistant.components.weather import (
+    DOMAIN as WEATHER_DOMAIN,
+    SERVICE_GET_FORECASTS,
+)
+from homeassistant.const import ATTR_ENTITY_ID
+from homeassistant.core import HomeAssistant
+
+from tests.common import MockConfigEntry
+
+
+@pytest.mark.freeze_time("2021-11-24T03:00:00+00:00")
+async def test_forecast_service(
+    hass: HomeAssistant,
+    mock_config_entry: MockConfigEntry,
+    mock_open_meteo: AsyncMock,
+    snapshot: SnapshotAssertion,
+) -> None:
+    """Test forecast service."""
+    mock_config_entry.add_to_hass(hass)
+    await hass.config_entries.async_setup(mock_config_entry.entry_id)
+    await hass.async_block_till_done()
+
+    response = await hass.services.async_call(
+        WEATHER_DOMAIN,
+        SERVICE_GET_FORECASTS,
+        {ATTR_ENTITY_ID: "weather.home", "type": "daily"},
+        blocking=True,
+        return_response=True,
+    )
+    assert response == snapshot(name="forecast_daily")
+
+    response = await hass.services.async_call(
+        WEATHER_DOMAIN,
+        SERVICE_GET_FORECASTS,
+        {ATTR_ENTITY_ID: "weather.home", "type": "hourly"},
+        blocking=True,
+        return_response=True,
+    )
+    assert response == snapshot(name="forecast_hourly")