fix(utils): compute midnight_utc with correct DST offset

On DST transition days, datetime.replace(hour=0) preserves the current
UTC offset instead of the offset in effect at midnight. For example on
UK spring-forward day, now at 14:00+01:00 produces midnight as
00:00+01:00 (actually 23:00 UTC previous day) instead of the correct
00:00+00:00. This shifts every rate minute index by 60 minutes, breaking
charge/discharge schedules.

Add a local_midnight() helper in utils.py that strips tzinfo and
re-localises via pytz.localize(), which resolves the correct offset for
the given date/time. For fixed-offset timezones (no DST) the simple
replace() path is preserved.

Replace all broken .replace(hour=0) patterns across predbat.py,
gecloud.py, fox.py, octopus.py, solis.py, predheat.py, and the
test_execute.py mock with calls to local_midnight().

Add a DST-specific test in test_fetch_octopus_rates.py verifying that
rates with mixed +00:00/+01:00 offsets land at the correct minute
indices.

Author: bpinto

apps/predbat/fox.py

@@ -25,6 +25,7 @@
 import random
 from component_base import ComponentBase
 from oauth_mixin import OAuthMixin
+from utils import local_midnight
 
 # Define TIME_FORMAT_HA locally to avoid dependency issues
 TIME_FORMAT_HA = "%Y-%m-%dT%H:%M:%S%z"
@@ -1712,7 +1713,7 @@ def __init__(self):
         self.now_utc = datetime.now(self.local_tz)
         self.prefix = "predbat"
         self.args = {}
-        self.midnight_utc = datetime.now().replace(hour=0, minute=0, second=0, microsecond=0)
+        self.midnight_utc = local_midnight(datetime.now(self.local_tz), self.local_tz)
         self.minutes_now = self.now_utc.hour * 60 + self.now_utc.minute
         self.entities = {}
 

apps/predbat/gecloud.py

@@ -14,7 +14,7 @@
 
 import aiohttp
 from datetime import timedelta, datetime
-from utils import str2time, dp1
+from utils import str2time, dp1, local_midnight
 from predbat_metrics import record_api_call
 import asyncio
 import json
@@ -1708,7 +1708,7 @@ def __init__(self):
         self.now_utc = datetime.now(self.local_tz)
         self.prefix = "predbat"
         self.args = {}
-        self.midnight_utc = datetime.now(self.local_tz).replace(hour=0, minute=0, second=0, microsecond=0)
+        self.midnight_utc = local_midnight(datetime.now(self.local_tz), self.local_tz)
         self.minutes_now = self.now_utc.hour * 60 + self.now_utc.minute
         self.entities = {}
         self.config_root = "./temp_gecloud"

apps/predbat/octopus.py

@@ -19,7 +19,7 @@
 from datetime import datetime, timedelta, timezone
 from predbat_metrics import record_api_call
 from const import TIME_FORMAT, TIME_FORMAT_OCTOPUS
-from utils import str2time, minutes_to_time, dp1, dp2, dp4, minute_data
+from utils import str2time, minutes_to_time, dp1, dp2, dp4, minute_data, local_midnight
 from component_base import ComponentBase
 import aiohttp
 import json
@@ -2718,7 +2718,7 @@ def __init__(self):
         self.now_utc = datetime.now(self.local_tz)
         self.prefix = "predbat"
         self.args = {}
-        self.midnight_utc = datetime.now(self.local_tz).replace(hour=0, minute=0, second=0, microsecond=0)
+        self.midnight_utc = local_midnight(datetime.now(self.local_tz), self.local_tz)
         self.minutes_now = self.now_utc.hour * 60 + self.now_utc.minute
         self.entities = {}
         self.config_root = "./temp_octopus"

apps/predbat/predbat.py

@@ -70,7 +70,7 @@
 )
 from config import APPS_SCHEMA, CONFIG_ITEMS
 from prediction import reset_prediction_globals
-from utils import minutes_since_yesterday, dp1, dp2, dp3
+from utils import minutes_since_yesterday, dp1, dp2, dp3, local_midnight
 from predheat import PredHeat
 from octopus import Octopus
 from energydataservice import Energidataservice
@@ -709,7 +709,7 @@ def update_time(self, print=True):
         self.now_utc = now_utc
         self.now = now
         self.midnight = now.replace(hour=0, minute=0, second=0, microsecond=0)
-        self.midnight_utc = now_utc.replace(hour=0, minute=0, second=0, microsecond=0)
+        self.midnight_utc = local_midnight(now_utc, self.local_tz)
 
         self.difference_minutes = minutes_since_yesterday(now)
         self.minutes_now = int((now - self.midnight).seconds / 60 / PREDICT_STEP) * PREDICT_STEP

apps/predbat/predheat.py

@@ -21,7 +21,7 @@
 
 from datetime import datetime, timedelta
 import pytz
-from utils import str2time, dp2, dp3, minute_data
+from utils import str2time, dp2, dp3, minute_data, local_midnight
 
 from const import TIME_FORMAT
 
@@ -533,7 +533,7 @@ def update_pred(self, scheduled):
         self.forecast_days = self.get_arg("forecast_days", 2, domain="predheat")
         self.forecast_minutes = self.forecast_days * 60 * 24
         self.midnight = now.replace(hour=0, minute=0, second=0, microsecond=0)
-        self.midnight_utc = now_utc.replace(hour=0, minute=0, second=0, microsecond=0)
+        self.midnight_utc = local_midnight(now_utc, local_tz)
         self.minutes_now = int((now - self.midnight).seconds / 60 / PREDICT_STEP) * PREDICT_STEP
         self.metric_future_rate_offset_import = 0
 

apps/predbat/solis.py

@@ -17,6 +17,7 @@
 from datetime import datetime, timedelta, UTC
 from predbat_metrics import record_api_call
 from component_base import ComponentBase
+from utils import local_midnight
 
 
 # API Endpoints
@@ -2869,7 +2870,7 @@ def __init__(self):
         self.now_utc = datetime.now(self.local_tz)
         self.prefix = "predbat"
         self.args = {}
-        self.midnight_utc = datetime.now().replace(hour=0, minute=0, second=0, microsecond=0)
+        self.midnight_utc = local_midnight(datetime.now(self.local_tz), self.local_tz)
         self.minutes_now = self.now_utc.hour * 60 + self.now_utc.minute
         self.entities = {}
 

apps/predbat/tests/test_execute.py

@@ -9,11 +9,14 @@
 # pylint: disable=attribute-defined-outside-init
 
 from tests.test_infra import reset_inverter
-from utils import calc_percent_limit
+from utils import calc_percent_limit, local_midnight
 
 
 class ActiveTestInverter:
-    def __init__(self, id, soc_kw, soc_max, now_utc):
+    """Mock inverter for execute tests."""
+
+    def __init__(self, id, soc_kw, soc_max, now_utc, local_tz=None):
+        """Initialise mock inverter."""
         self.soc_target = -1
         self.id = id
         self.isCharging = False
@@ -53,7 +56,7 @@ def __init__(self, id, soc_kw, soc_max, now_utc):
         self.battery_rate_max_discharge = 1 / 60.0
         self.reserve_max = 100.0
         self.now_utc = now_utc
-        self.midnight_utc = now_utc.replace(hour=0, minute=0, second=0, microsecond=0)
+        self.midnight_utc = local_midnight(now_utc, local_tz)
         self.count_register_writes = 0
         self.charge_window = []
         self.charge_limits = []
@@ -424,7 +427,7 @@ def run_execute_tests(my_predbat):
     export_limits_best3 = [50.5]
     export_limits_best_frz = [99]
 
-    inverters = [ActiveTestInverter(0, 0, 10.0, my_predbat.now_utc), ActiveTestInverter(1, 0, 10.0, my_predbat.now_utc)]
+    inverters = [ActiveTestInverter(0, 0, 10.0, my_predbat.now_utc, my_predbat.local_tz), ActiveTestInverter(1, 0, 10.0, my_predbat.now_utc, my_predbat.local_tz)]
     my_predbat.inverters = inverters
     my_predbat.args["num_inverters"] = 2
 
@@ -1249,7 +1252,7 @@ def run_execute_tests(my_predbat):
     my_predbat.debug_enable = False
 
     # Reset inverters
-    inverters = [ActiveTestInverter(0, 0, 10.0, my_predbat.now_utc), ActiveTestInverter(1, 0, 10.0, my_predbat.now_utc)]
+    inverters = [ActiveTestInverter(0, 0, 10.0, my_predbat.now_utc, my_predbat.local_tz), ActiveTestInverter(1, 0, 10.0, my_predbat.now_utc, my_predbat.local_tz)]
     my_predbat.inverters = inverters
 
     failed |= run_execute_test(my_predbat, "calibration", in_calibration=True, assert_status="Calibration", assert_charge_time_enable=False, assert_reserve=0, assert_soc_target=100)
@@ -1758,7 +1761,7 @@ def run_execute_tests(my_predbat):
     failed |= run_execute_test(my_predbat, "charge_later2", charge_window_best=charge_window_best6, charge_limit_best=charge_limit_best, assert_charge_time_enable=False, set_charge_window=True, set_export_window=True, assert_status="Demand")
     failed |= run_execute_test(my_predbat, "no_charge5", set_charge_window=True, set_export_window=True, assert_immediate_soc_target=0)
     # Reset inverters
-    inverters = [ActiveTestInverter(0, 0, 10.0, my_predbat.now_utc), ActiveTestInverter(1, 0, 10.0, my_predbat.now_utc)]
+    inverters = [ActiveTestInverter(0, 0, 10.0, my_predbat.now_utc, my_predbat.local_tz), ActiveTestInverter(1, 0, 10.0, my_predbat.now_utc, my_predbat.local_tz)]
     my_predbat.inverters = inverters
 
     failed |= run_execute_test(my_predbat, "no_discharge", export_window_best=export_window_best, export_limits_best=export_limits_best, assert_reserve=-1)

apps/predbat/tests/test_fetch_octopus_rates.py

@@ -244,6 +244,74 @@ def test_fetch_octopus_rates(my_predbat):
     else:
         print("Test 8 passed - sensor with no attributes returns empty dict")
 
+    # Test 9: DST transition day - mixed timezone offsets in rate data
+    print("*** Test 9: DST spring-forward day with mixed +00:00 / +01:00 offsets")
+
+    # On UK spring-forward day (last Sunday in March), clocks go from GMT (+00:00)
+    # to BST (+01:00) at 01:00 GMT.  The Octopus integration delivers rates before
+    # the transition with +00:00 and rates after with +01:00.
+    # midnight_utc must carry the correct offset (+00:00) so that minute indices
+    # are computed correctly for both sets of rates.
+
+    import pytz
+
+    london_tz = pytz.timezone("Europe/London")
+    # Simulate "now" at 14:00 BST on spring-forward day
+    my_predbat.local_tz = london_tz
+    my_predbat.midnight_utc = london_tz.localize(datetime(2026, 3, 29, 0, 0, 0))
+    my_predbat.forecast_days = 2
+
+    entity_id_dst = "sensor.metric_octopus_import_dst"
+    dst_rates = [
+        # Pre-DST rates (GMT, +00:00)
+        {"start": "2026-03-29T00:00:00+00:00", "end": "2026-03-29T00:30:00+00:00", "value": 0.04138},
+        {"start": "2026-03-29T00:30:00+00:00", "end": "2026-03-29T01:00:00+00:00", "value": 0.04135},
+        # Post-DST rates (BST, +01:00) — 02:00 BST = 01:00 UTC
+        {"start": "2026-03-29T02:00:00+01:00", "end": "2026-03-29T02:30:00+01:00", "value": 0.04000},
+        {"start": "2026-03-29T02:30:00+01:00", "end": "2026-03-29T03:00:00+01:00", "value": 0.03991},
+    ]
+
+    my_predbat.ha_interface.dummy_items[entity_id_dst] = {
+        "state": "0.04",
+        "raw_today": dst_rates,
+    }
+
+    rate_data = my_predbat.fetch_octopus_rates(entity_id_dst)
+
+    if not rate_data:
+        print("ERROR: No rate data returned for DST test")
+        failed = True
+    else:
+        # 00:00+00:00 is minute 0 from midnight
+        expected_min0 = 0.04138 * 100  # scale=100 for start/end format
+        if 0 not in rate_data:
+            print("ERROR: DST test - missing rate at minute 0")
+            failed = True
+        elif abs(rate_data[0] - expected_min0) > 0.01:
+            print("ERROR: DST test - minute 0 expected {}, got {}".format(expected_min0, rate_data[0]))
+            failed = True
+
+        # 02:00+01:00 = 01:00 UTC = minute 60 from midnight (+00:00)
+        expected_min60 = 0.04000 * 100
+        if 60 not in rate_data:
+            print("ERROR: DST test - missing rate at minute 60 (02:00 BST = 01:00 UTC)")
+            failed = True
+        elif abs(rate_data[60] - expected_min60) > 0.01:
+            print("ERROR: DST test - minute 60 expected {}, got {}".format(expected_min60, rate_data[60]))
+            failed = True
+
+        # 02:30+01:00 = 01:30 UTC = minute 90 from midnight (+00:00)
+        expected_min90 = 0.03991 * 100
+        if 90 not in rate_data:
+            print("ERROR: DST test - missing rate at minute 90 (02:30 BST = 01:30 UTC)")
+            failed = True
+        elif abs(rate_data[90] - expected_min90) > 0.01:
+            print("ERROR: DST test - minute 90 expected {}, got {}".format(expected_min90, rate_data[90]))
+            failed = True
+
+        if 0 in rate_data and 60 in rate_data and 90 in rate_data:
+            print("Test 9 passed - DST mixed-offset rates placed at correct minutes")
+
     # Restore original values
     my_predbat.forecast_days = old_forecast_days
     my_predbat.midnight_utc = old_midnight_utc

apps/predbat/utils.py

@@ -876,6 +876,25 @@ def minutes_to_time(updated, now):
     return minutes
 
 
+def local_midnight(dt_aware, tz=None):
+    """Return midnight on the same local date as *dt_aware*, correctly localised.
+
+    ``datetime.replace(hour=0)`` preserves the original UTC offset, which is
+    wrong on DST-transition days (e.g. spring-forward: the current offset is
+    +01:00 but midnight was still +00:00).  Stripping the tzinfo and
+    re-localising lets pytz pick the offset that was actually in effect at
+    midnight.
+
+    *tz* should be a pytz timezone (has ``.localize()``).  When *tz* is
+    ``None`` or a fixed-offset timezone the simple ``replace()`` path is used,
+    which is safe because fixed-offset zones never change their UTC offset.
+    """
+    if tz is not None and hasattr(tz, "localize"):
+        naive_midnight = dt_aware.replace(hour=0, minute=0, second=0, microsecond=0, tzinfo=None)
+        return tz.localize(naive_midnight)
+    return dt_aware.replace(hour=0, minute=0, second=0, microsecond=0)
+
+
 def str2time(str):
     if "." in str:
         tdata = datetime.strptime(str, TIME_FORMAT_SECONDS)