trim the datetime helpers and comments from review

mobile/lib/infrastructure/repositories/remote_album.repository.dart

@@ -328,7 +328,7 @@ class DriftRemoteAlbumRepository extends DriftDatabaseRepository {
             isFavorite: Value(source.isFavorite),
             visibility: const Value(AssetVisibility.timeline),
             isEdited: Value(source.isEdited),
-            groupDate: Value(remoteGroupDate(null, source.createdAt)),
+            groupDate: Value(timelineGroupDate(source.createdAt.toLocal())),
           ),
           mode: InsertMode.insertOrIgnore,
         );

mobile/lib/infrastructure/repositories/remote_asset.repository.dart

@@ -12,6 +12,7 @@ import 'package:immich_mobile/infrastructure/entities/stack.entity.drift.dart';
 import 'package:immich_mobile/infrastructure/repositories/db.repository.dart';
 import 'package:immich_mobile/utils/datetime_helpers.dart';
 import 'package:immich_mobile/utils/option.dart';
+import 'package:immich_mobile/utils/timezone.dart';
 import 'package:maplibre_gl/maplibre_gl.dart';
 
 class RemoteAssetRepository extends DriftDatabaseRepository {
@@ -293,9 +294,8 @@ class RemoteAssetRepository extends DriftDatabaseRepository {
   }
 
   Future<void> updateDateTime(List<String> ids, DateTime dateTime, {String? timeZone}) {
-    // the picker value arrives as UTC; its wall day lives in the offset
-    final offset = tryParseUtcOffset(timeZone);
-    final groupDate = timelineGroupDate(offset != null ? dateTime.toUtc().add(offset) : dateTime);
+    final (adjusted, _) = applyTimezoneOffset(dateTime: dateTime, timeZone: timeZone);
+    final groupDate = timelineGroupDate(adjusted);
     return _db.batch((batch) async {
       for (final id in ids) {
         batch.update(

mobile/lib/infrastructure/repositories/sync_stream.repository.dart

@@ -200,6 +200,7 @@ class SyncStreamRepository extends DriftDatabaseRepository {
     try {
       await _db.batch((batch) {
         for (final asset in data) {
+          final groupDate = asset.localDateTime ?? asset.fileCreatedAt?.toLocal();
           final companion = RemoteAssetEntityCompanion(
             name: Value(asset.originalFileName),
             type: Value(asset.type.toAssetType()),
@@ -211,9 +212,7 @@ class SyncStreamRepository extends DriftDatabaseRepository {
             isFavorite: Value(asset.isFavorite),
             ownerId: Value(asset.ownerId),
             localDateTime: Value(asset.localDateTime),
-            groupDate: asset.localDateTime == null && asset.fileCreatedAt == null
-                ? const Value.absent()
-                : Value(remoteGroupDate(asset.localDateTime, asset.fileCreatedAt!)),
+            groupDate: groupDate == null ? const Value.absent() : Value(timelineGroupDate(groupDate)),
             thumbHash: Value(asset.thumbhash),
             deletedAt: Value(asset.deletedAt),
             visibility: Value(asset.visibility.toAssetVisibility()),
@@ -242,6 +241,7 @@ class SyncStreamRepository extends DriftDatabaseRepository {
     try {
       await _db.batch((batch) {
         for (final asset in data) {
+          final groupDate = asset.localDateTime ?? asset.fileCreatedAt?.toLocal();
           final companion = RemoteAssetEntityCompanion(
             name: Value(asset.originalFileName),
             type: Value(asset.type.toAssetType()),
@@ -253,9 +253,7 @@ class SyncStreamRepository extends DriftDatabaseRepository {
             isFavorite: Value(asset.isFavorite),
             ownerId: Value(asset.ownerId),
             localDateTime: Value(asset.localDateTime),
-            groupDate: asset.localDateTime == null && asset.fileCreatedAt == null
-                ? const Value.absent()
-                : Value(remoteGroupDate(asset.localDateTime, asset.fileCreatedAt!)),
+            groupDate: groupDate == null ? const Value.absent() : Value(timelineGroupDate(groupDate)),
             thumbHash: Value(asset.thumbhash),
             deletedAt: Value(asset.deletedAt),
             visibility: Value(asset.visibility.toAssetVisibility()),

mobile/lib/utils/datetime_helpers.dart

@@ -18,25 +18,6 @@ DateTime? tryFromSecondsSinceEpoch(int? secondsSinceEpoch, {bool isUtc = false})
   }
 }
 
-// Uses the held components, so convert with toLocal() first for instants.
-// Dates sqlite cannot read (year outside 1..9999) fall to null instead of a mangled day
-String? timelineGroupDate(DateTime value) {
-  if (value.year < 1 || value.year > 9999) {
-    return null;
-  }
-  return value.toIso8601String().split('T').first;
-}
-
-// 'UTC+14:00' style, from the date picker path
-Duration? tryParseUtcOffset(String? value) {
-  final match = value == null ? null : RegExp(r'^UTC([+-])(\d{2}):(\d{2})$').firstMatch(value);
-  if (match == null) {
-    return null;
-  }
-  final minutes = int.parse(match[2]!) * 60 + int.parse(match[3]!);
-  return Duration(minutes: match[1] == '-' ? -minutes : minutes);
-}
-
-// group_date for remote rows: wall day when known, else the local day of createdAt
-String? remoteGroupDate(DateTime? localDateTime, DateTime createdAt) =>
-    (localDateTime != null ? timelineGroupDate(localDateTime) : null) ?? timelineGroupDate(createdAt.toLocal());
+// no DateFormat: it tracks Intl.defaultLocale and needs locale init per isolate
+String timelineGroupDate(DateTime value) =>
+    '${value.year.toString().padLeft(4, '0')}-${value.month.toString().padLeft(2, '0')}-${value.day.toString().padLeft(2, '0')}';

mobile/lib/utils/migration.dart

@@ -48,8 +48,6 @@ Future<void> migrateDatabaseIfNeeded(Drift drift) async {
 
 Future<void> _migrateTo28(Drift drift) => backfillAssetGroupDates(drift);
 
-// Store-level on purpose: runs after the date heals in this chain, so group_date is
-// computed from the corrected values. STRFTIME drops dates sqlite cannot read.
 Future<void> backfillAssetGroupDates(Drift drift) async {
   await drift.customStatement(
     "UPDATE remote_asset_entity SET group_date = COALESCE(STRFTIME('%Y-%m-%d', local_date_time), STRFTIME('%Y-%m-%d', created_at, 'localtime'))",

mobile/test/drift/main/migration_test.dart

@@ -41,8 +41,6 @@ void main() {
   });
 
   group('v32 group_date backfill', () {
-    // 26479 class: platform handed us a date sqlite cannot read. the bucket stream
-    // must not die, and the broken row must vanish from buckets and assets alike.
     test(
       'garbage created_at survives migration and is filtered from the timeline',
       () async {
@@ -80,8 +78,6 @@ void main() {
       },
     );
 
-    // The drift migration only adds the column; the backfill is a store-level step so a
-    // created_at heal (29193) that runs before it actually lands in group_date.
     test('a created_at heal before the backfill lands in the header day', () async {
       await initializeDateFormatting();
       final schema = await verifier.schemaAt(31);
@@ -98,7 +94,6 @@ void main() {
       final db = Drift(schema.newConnection());
       await verifier.migrateAndValidate(db, 32);
 
-      // 29193's store-level heal
       await db.customStatement(
         "UPDATE local_asset_entity SET created_at = updated_at WHERE julianday(created_at) > julianday(updated_at)",
       );

mobile/test/infrastructure/repositories/merged_asset_drift_test.dart

@@ -49,7 +49,7 @@ void main() {
           updatedAt: Value(createdAt),
           uploadedAt: Value(createdAt),
           localDateTime: Value(localDateTime),
-          groupDate: Value(remoteGroupDate(localDateTime, createdAt)),
+          groupDate: Value(timelineGroupDate(localDateTime ?? createdAt.toLocal())),
         ),
       );
 
@@ -72,8 +72,7 @@ void main() {
         .insert(LocalAlbumAssetEntityCompanion.insert(assetId: id, albumId: _albumId));
   }
 
-  // Mirrors how the timeline pairs headers with tiles: buckets only carry a count, the assets
-  // come from one flat list that is addressed by the running offset of the previous buckets.
+  // the timeline pairs headers to assets by running offset into one flat list
   Future<List<(String, String)>> headerForEachAsset(GroupAssetsBy groupBy) async {
     final buckets = await db.mergedAssetDrift.mergedBucket(groupBy: groupBy.index, userIds: [_userId]).get();
     final assets = await db.mergedAssetDrift.mergedAsset(userIds: [_userId], limit: (_) => Limit(1000, 0)).get();
@@ -89,8 +88,7 @@ void main() {
     return pairs;
   }
 
-  // Regression for #29864: buckets group by localDateTime but assets were ordered by createdAt,
-  // so a header minted from one asset's localDateTime was rendered above a different asset.
+  // #29864: buckets grouped by localDateTime while assets sorted by createdAt
   group('mergedAsset ordering matches mergedBucket grouping', () {
     Future<void> seedGhost() async {
       await insertRemote('asset-a', createdAt: DateTime(2026, 4, 26, 10));
@@ -122,9 +120,6 @@ void main() {
       expect(await headerForEachAsset(GroupAssetsBy.day), [('asset-b', '2026-04-26'), ('asset-a', '2026-04-26')]);
     });
 
-    // Web-consistency: the server groups on the date only and sorts within a day by
-    // createdAt, so two assets whose wall clock and createdAt disagree within the same
-    // day must come out in createdAt order.
     test('same-day assets order by createdAt like the web timeline', () async {
       await insertRemote(
         'shot-late-upload-early',
@@ -163,8 +158,6 @@ void main() {
     expect(await headerForEachAsset(GroupAssetsBy.month), [('may', '2024-05'), ('april', '2024-04')]);
   });
 
-  // Store-migration order: the 29193 heal runs first, the group_date backfill after it,
-  // so the corrected date is what lands under the header.
   test('a created_at heal before the backfill lands in the header day', () async {
     await db
         .into(db.localAssetEntity)