Properly handle timezones

This should (hopefully 🤞) take care of problems saving entries with timezone
information.

test/helpers/date_test.dart

@@ -1,31 +0,0 @@
-/*
- * This file is part of wger Workout Manager <https://github.com/wger-project>.
- * Copyright (C) wger Team
- *
- * wger Workout Manager is free software: you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * wger Workout Manager is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU Affero General Public License for more details.
- *
- * You should have received a copy of the GNU Affero General Public License
- * along with this program.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-import 'package:flutter_test/flutter_test.dart';
-import 'package:wger/helpers/date.dart';
-
-void main() {
-  group('getDateTimeFromDateAndTime', () {
-    test('should correctly generate a DateTime', () {
-      expect(
-        getDateTimeFromDateAndTime('2025-05-16', '17:02'),
-        DateTime(2025, 5, 16, 17, 2),
-      );
-    });
-  });
-}

test/helpers/json_test.dart

@@ -57,13 +57,20 @@ void main() {
       });
     });
 
-    group('dateToIsoWithTimezone', () {
+    group('Iso8601 and timezones', () {
       test('should format DateTime to a string with timezone', () {
         expect(
           dateToUtcIso8601(DateTime.parse('2025-05-16T18:15:00+02:00')),
           '2025-05-16T16:15:00.000Z',
         );
       });
+
+      test('should convert an iso8601 datetime to local', () {
+        expect(
+          utcIso8601ToLocalDate('2025-11-18T18:15:00+08:00'),
+          DateTime.parse('2025-11-18T11:15:00.000'),
+        );
+      });
     });
 
     group('stringToTime', () {

test/weight/weight_model_test.dart

@@ -23,13 +23,13 @@ void main() {
   group('fetchPost', () {
     test('Test that the weight entries are correctly converted to json', () {
       expect(
-        WeightEntry(id: 1, weight: 80, date: DateTime(2020, 12, 31, 12, 34)).toJson(),
-        {'id': 1, 'weight': '80', 'date': '2020-12-31T12:34:00.000'},
+        WeightEntry(id: 1, weight: 80, date: DateTime.utc(2020, 12, 31, 12, 34)).toJson(),
+        {'id': 1, 'weight': '80', 'date': '2020-12-31T12:34:00.000Z'},
       );
 
       expect(
-        WeightEntry(id: 2, weight: 70.2, date: DateTime(2020, 12, 01)).toJson(),
-        {'id': 2, 'weight': '70.2', 'date': '2020-12-01T00:00:00.000'},
+        WeightEntry(id: 2, weight: 70.2, date: DateTime.utc(2020, 12, 01)).toJson(),
+        {'id': 2, 'weight': '70.2', 'date': '2020-12-01T00:00:00.000Z'},
       );
     });
 
@@ -53,12 +53,11 @@ void main() {
   group('model', () {
     test('Test the individual values from the model', () {
       WeightEntry weightModel;
-      //_weightModel = WeightEntry();
-      weightModel = WeightEntry(id: 1, weight: 80, date: DateTime(2020, 10, 01));
+      weightModel = WeightEntry(id: 1, weight: 80, date: DateTime.utc(2020, 10, 01));
 
       expect(weightModel.id, 1);
       expect(weightModel.weight, 80);
-      expect(weightModel.date, DateTime(2020, 10, 01));
+      expect(weightModel.date, DateTime.utc(2020, 10, 01));
     });
   });
 }

test/weight/weight_provider_test.dart

@@ -72,14 +72,14 @@ void main() {
       when(mockBaseProvider.makeUrl(any, query: anyNamed('query'))).thenReturn(uri);
       when(
         mockBaseProvider.post(
-          {'id': null, 'weight': '80', 'date': '2021-01-01T00:00:00.000'},
+          {'id': null, 'weight': '80', 'date': '2021-01-01T00:00:00.000Z'},
           uri,
         ),
       ).thenAnswer((_) => Future.value({'id': 25, 'date': '2021-01-01', 'weight': '80'}));
 
       // Act
       final BodyWeightProvider provider = BodyWeightProvider(mockBaseProvider);
-      final WeightEntry weightEntry = WeightEntry(date: DateTime(2021, 1, 1), weight: 80);
+      final WeightEntry weightEntry = WeightEntry(date: DateTime.utc(2021, 1, 1), weight: 80);
       final WeightEntry weightEntryNew = await provider.addEntry(weightEntry);
 
       // Assert