Fix per-database timezone datetime handling

mssql/operations.py

@@ -31,13 +31,17 @@ def max_in_list_size(self):
 
     def _convert_field_to_tz(self, field_name, tzname):
         if tzname and settings.USE_TZ and self.connection.timezone_name != tzname:
-            offset = self._get_utcoffset(tzname)
+            target_offset = self._get_utcoffset(tzname)
+            connection_offset = self._get_utcoffset(self.connection.timezone_name)
+            offset = target_offset - connection_offset
             field_name = 'DATEADD(second, %d, %s)' % (offset, field_name)
         return field_name
 
     def _convert_sql_to_tz(self, sql, params, tzname):
         if tzname and settings.USE_TZ and self.connection.timezone_name != tzname:
-            offset = self._get_utcoffset(tzname)
+            target_offset = self._get_utcoffset(tzname)
+            connection_offset = self._get_utcoffset(self.connection.timezone_name)
+            offset = target_offset - connection_offset
             sql = 'DATEADD(second, %d, %s)' % (offset, sql)
         return sql, params
 
@@ -614,10 +618,11 @@ def adapt_datetimefield_value(self, value):
 
         if timezone.is_aware(value):
             if settings.USE_TZ:
-                # When support for time zones is enabled, Django stores datetime information
-                # in UTC in the database and uses time-zone-aware objects internally
-                # source: https://docs.djangoproject.com/en/dev/topics/i18n/timezones/#overview
-                value = value.astimezone(datetime.timezone.utc)
+                # SQL Server's datetime2 type doesn't preserve time zone
+                # information. Store aware values as naive datetimes in the
+                # connection's configured time zone so the read and write paths
+                # use the same convention.
+                value = timezone.make_naive(value, self.connection.timezone)
             else:
                 # When USE_TZ is False, settings.TIME_ZONE is the time zone in
                 # which Django will store all datetimes

testapp/tests/test_timezones.py

@@ -2,12 +2,33 @@
 # Licensed under the BSD license.
 
 import datetime
+from contextlib import contextmanager
+
 from django.db import connection
 from django.test import TestCase
 from django.test.utils import override_settings
 
 from ..models import TimeZone
 
+
+@contextmanager
+def override_database_connection_timezone(timezone):
+    original_timezone = connection.settings_dict['TIME_ZONE']
+    try:
+        connection.settings_dict['TIME_ZONE'] = timezone
+        connection.timezone
+        del connection.timezone
+        connection.timezone_name
+        del connection.timezone_name
+        yield
+    finally:
+        connection.settings_dict['TIME_ZONE'] = original_timezone
+        connection.timezone
+        del connection.timezone
+        connection.timezone_name
+        del connection.timezone_name
+
+
 class TestDateTimeField(TestCase):
 
     def test_iso_week_day(self):
@@ -23,6 +44,47 @@ def test_iso_week_day(self):
         for k, v in days.items():
             self.assertSequenceEqual(TimeZone.objects.filter(date__iso_week_day=k), [v])
 
+    @override_settings(USE_TZ=True)
+    def test_adapt_datetimefield_value_uses_database_timezone(self):
+        value = datetime.datetime(2024, 1, 15, 12, tzinfo=datetime.timezone.utc)
+
+        with override_database_connection_timezone('Europe/Berlin'):
+            adapted = connection.ops.adapt_datetimefield_value(value)
+
+        self.assertEqual(adapted, datetime.datetime(2024, 1, 15, 13))
+        self.assertIsNone(adapted.tzinfo)
+
+    @override_settings(USE_TZ=True)
+    def test_datetime_sql_conversion_uses_database_timezone(self):
+        with override_database_connection_timezone('Asia/Bangkok'):
+            converted_field = connection.ops._convert_field_to_tz(
+                '[date]', 'Africa/Nairobi'
+            )
+            converted_sql, params = connection.ops._convert_sql_to_tz(
+                '[date]', [], 'Africa/Nairobi'
+            )
+
+        expected = 'DATEADD(second, -14400, [date])'
+        self.assertEqual(converted_field, expected)
+        self.assertEqual(converted_sql, expected)
+        self.assertEqual(params, [])
+
+    @override_settings(USE_TZ=True, TIME_ZONE='Africa/Nairobi')
+    def test_datetime_round_trip_uses_database_timezone(self):
+        value = datetime.datetime(2024, 7, 15, 20, 10, tzinfo=datetime.timezone.utc)
+        database_timezone = datetime.timezone(datetime.timedelta(hours=7))
+
+        with override_database_connection_timezone('Asia/Bangkok'):
+            obj = TimeZone.objects.create(date=value)
+            retrieved = TimeZone.objects.get(pk=obj.pk)
+            by_exact_datetime = TimeZone.objects.get(date=value)
+            by_local_date = TimeZone.objects.get(date__date=datetime.date(2024, 7, 15))
+
+        self.assertEqual(retrieved.date, value.astimezone(database_timezone))
+        self.assertEqual(by_exact_datetime, obj)
+        self.assertEqual(by_local_date, obj)
+
+
 class TestDateTimeToDateTimeOffsetMigration(TestCase):
 
     def setUp(self):