diff --git a/mobile/lib/features/channels/channel_detail_page/message_list.dart b/mobile/lib/features/channels/channel_detail_page/message_list.dart
index 804fab87af..d90b4bc167 100644
--- a/mobile/lib/features/channels/channel_detail_page/message_list.dart
+++ b/mobile/lib/features/channels/channel_detail_page/message_list.dart
@@ -56,7 +56,7 @@ class _MessageList extends HookConsumerWidget {
       localMessageSendAnimationProvider(channelId),
     );
     final displayEntries = groupMembershipTimelineEntries(entries);
-    final calendarDays = TimelineCalendarDays();
+    final calendarDays = ref.watch(timelineCalendarDaysFactoryProvider)();
     final itemScrollController = useMemoized(ItemScrollController.new);
     final itemPositionsListener = useMemoized(ItemPositionsListener.create);
     final stickyDayTimestamp = useValueNotifier<int?>(null);
@@ -976,7 +976,7 @@ class _MessageList extends HookConsumerWidget {
                             startsDay: showDayDivider,
                             endsDay:
                                 nextMessage != null &&
-                                !isSameDay(
+                                !calendarDays.isSameDay(
                                   message.createdAt,
                                   nextMessage.createdAt,
                                 ),
@@ -997,7 +997,7 @@ class _MessageList extends HookConsumerWidget {
                             startsDay: showDayDivider,
                             endsDay:
                                 nextMessage != null &&
-                                !isSameDay(
+                                !calendarDays.isSameDay(
                                   message.createdAt,
                                   nextMessage.createdAt,
                                 ),
diff --git a/mobile/lib/features/channels/date_formatters.dart b/mobile/lib/features/channels/date_formatters.dart
index b165394ed4..603814c998 100644
--- a/mobile/lib/features/channels/date_formatters.dart
+++ b/mobile/lib/features/channels/date_formatters.dart
@@ -1,4 +1,5 @@
 import 'package:flutter/foundation.dart';
+import 'package:hooks_riverpod/hooks_riverpod.dart';
 import 'package:intl/intl.dart';
 
 // Re-export shortPubkey so existing callers continue to compile.
@@ -107,6 +108,12 @@ class TimelineCalendarDays {
   bool isSameDay(int a, int b) => _day(a) == _day(b);
 }
 
+/// Creates a fresh calendar cache for each timeline build.
+final timelineCalendarDaysFactoryProvider =
+    Provider<TimelineCalendarDays Function()>(
+      (ref) => TimelineCalendarDays.new,
+    );
+
 /// Returns a compact relative time string like "just now", "5m ago", "3h ago",
 /// "2d ago", or a short date for older timestamps.
 String relativeTime(int unixSeconds) {
diff --git a/mobile/lib/features/channels/thread_detail_page/message_list.dart b/mobile/lib/features/channels/thread_detail_page/message_list.dart
index 1d2812f6c9..205c091247 100644
--- a/mobile/lib/features/channels/thread_detail_page/message_list.dart
+++ b/mobile/lib/features/channels/thread_detail_page/message_list.dart
@@ -1,6 +1,6 @@
 part of '../thread_detail_page.dart';
 
-class _ThreadMessageList extends HookWidget {
+class _ThreadMessageList extends HookConsumerWidget {
   final double headHeight;
   final ValueChanged<double> onHeadHeightChanged;
   final LaidOutViewport viewport;
@@ -144,8 +144,8 @@ class _ThreadMessageList extends HookWidget {
   }
 
   @override
-  Widget build(BuildContext context) {
-    final calendarDays = TimelineCalendarDays();
+  Widget build(BuildContext context, WidgetRef ref) {
+    final calendarDays = ref.watch(timelineCalendarDaysFactoryProvider)();
     final headLink = useMemoized(LayerLink.new);
     final loadingHeadLink = useMemoized(LayerLink.new);
     final headScrollPosition = useRef<ScrollPosition?>(null);
@@ -308,7 +308,7 @@ class _ThreadMessageList extends HookWidget {
                                     startsDay: showDayDivider,
                                     endsDay:
                                         nextReply != null &&
-                                        !isSameDay(
+                                        !calendarDays.isSameDay(
                                           reply.createdAt,
                                           nextReply.createdAt,
                                         ),
diff --git a/mobile/test/features/channels/channel_detail_page_test.dart b/mobile/test/features/channels/channel_detail_page_test.dart
index 55f02451a2..35edf8bbfb 100644
--- a/mobile/test/features/channels/channel_detail_page_test.dart
+++ b/mobile/test/features/channels/channel_detail_page_test.dart
@@ -92,6 +92,7 @@ import 'package:shared_preferences/shared_preferences.dart';
 import '../../helpers/thread_ids.dart';
 
 part 'thread_reply_refresh_cases.dart';
+part 'timeline_calendar_builder_cases.dart';
 part 'thread_title_capsule_cases.dart';
 part 'channel_detail_page_test/loading_review_tests.dart';
 part 'channel_detail_page_test/presence_tests.dart';
@@ -235,6 +236,7 @@ NostrEvent _edit({
 
 Widget _buildTestable({
   bool useRealChannelProviders = false,
+  TimelineCalendarDays Function()? calendarDaysFactory,
   ProfileNotifier? profileNotifier,
   String? signingPubkey,
   bool darkTheme = false,
@@ -303,6 +305,10 @@ Widget _buildTestable({
   return ProviderScope(
     retry: providerRetry ?? (disableRetries ? (_, _) => null : null),
     overrides: [
+      if (calendarDaysFactory != null)
+        timelineCalendarDaysFactoryProvider.overrideWithValue(
+          calendarDaysFactory,
+        ),
       if (sender != null) sendMessageProvider.overrideWithValue(sender),
       if (soloSender != null) ...[
         meSendMessageProvider.overrideWithValue(soloSender),
@@ -13642,134 +13648,7 @@ void main() {
       expect(observer.pushCount, initialPushCount + 1);
     });
 
-    testWidgets('thread shows day dividers when replies cross days', (
-      tester,
-    ) async {
-      final rootCreatedAt =
-          DateTime(2025, 1, 1, 12).toUtc().millisecondsSinceEpoch ~/ 1000;
-      final nextDayCreatedAt =
-          DateTime(2025, 1, 2, 12).toUtc().millisecondsSinceEpoch ~/ 1000;
-      final rootEvent = _textMsg(
-        id: threadIdThreadDashRoot,
-        pubkey: 'alice',
-        content: 'Thread root',
-        createdAt: rootCreatedAt,
-      );
-      final replies = [
-        _textMsg(
-          id: 'reply-same-day',
-          pubkey: 'bob',
-          content: 'Same day',
-          createdAt: rootCreatedAt + 60,
-          extraTags: const [
-            ['e', threadIdThreadDashRoot, '', 'reply'],
-          ],
-        ),
-        _textMsg(
-          id: 'reply-next-day',
-          pubkey: 'bob',
-          content: 'Next day',
-          createdAt: nextDayCreatedAt,
-          extraTags: const [
-            ['e', threadIdThreadDashRoot, '', 'reply'],
-          ],
-        ),
-      ];
-
-      await tester.pumpWidget(
-        _buildTestable(
-          messages: [rootEvent],
-          threadReplies: {threadIdThreadDashRoot: replies},
-          users: {
-            'alice': const UserProfile(pubkey: 'alice', displayName: 'Alice'),
-            'bob': const UserProfile(pubkey: 'bob', displayName: 'Bob'),
-          },
-        ),
-      );
-      await tester.pumpAndSettle();
-
-      final threadHead = formatTimeline([rootEvent]).single;
-      Navigator.of(tester.element(find.byType(ChannelDetailPage))).push(
-        MaterialPageRoute<void>(
-          builder: (_) => ThreadDetailPage(
-            threadHead: threadHead,
-            allMessages: [threadHead],
-            channelId: _channelId,
-            currentPubkey: 'self',
-            isMember: true,
-            isArchived: false,
-          ),
-        ),
-      );
-      await tester.pumpAndSettle();
-
-      expect(find.byType(DayDivider), findsNWidgets(2));
-      expect(
-        find.descendant(
-          of: find.byType(DayDivider),
-          matching: find.text(formatDayHeading(rootCreatedAt)),
-        ),
-        findsOneWidget,
-      );
-      expect(
-        find.descendant(
-          of: find.byType(DayDivider),
-          matching: find.text(formatDayHeading(nextDayCreatedAt)),
-        ),
-        findsOneWidget,
-      );
-      // The list runs top-down (head first), so tail spacing lives on the list
-      // and reply groups carry none.
-      final threadList = tester.widget<ScrollablePositionedList>(
-        find.byKey(const ValueKey('thread-message-list')),
-      );
-      expect(threadList.reverse, isFalse);
-      final threadComposerDockHeight = tester
-          .getSize(find.byKey(const ValueKey('thread-composer-dock')))
-          .height;
-      expect(threadList.padding!.bottom, Grid.xs + threadComposerDockHeight);
-      final newestThreadGroup = tester.widget<Padding>(
-        find.byKey(const ValueKey('thread-message-group-reply-next-day')),
-      );
-      expect(newestThreadGroup.padding, EdgeInsets.zero);
-
-      // The head sits above its replies rather than jammed against the
-      // composer, matching desktop's thread panel.
-      final headY = tester
-          .getTopLeft(
-            find.byKey(
-              const ValueKey('thread-message-group-$threadIdThreadDashRoot'),
-            ),
-          )
-          .dy;
-      final oldestReplyY = tester
-          .getTopLeft(
-            find.byKey(const ValueKey('thread-message-group-reply-same-day')),
-          )
-          .dy;
-      final newestReplyY = tester
-          .getTopLeft(
-            find.byKey(const ValueKey('thread-message-group-reply-next-day')),
-          )
-          .dy;
-      expect(headY, lessThan(oldestReplyY));
-      expect(oldestReplyY, lessThan(newestReplyY));
-      final threadTimestamp = tester.widget<Text>(
-        find.byKey(
-          const ValueKey('thread-message-timestamp-$threadIdThreadDashRoot'),
-        ),
-      );
-      expect(threadTimestamp.style?.fontSize, 11);
-      expect(
-        find.descendant(
-          of: find.byKey(
-            const ValueKey('thread-message-row-$threadIdThreadDashRoot'),
-          ),
-          matching: find.text('·'),
-        ),
-        findsNothing,
-      );
-    });
+    timelineCalendarBuilderTests();
 
     testWidgets('thread pins and updates the active date while scrolling', (
       tester,
diff --git a/mobile/test/features/channels/timeline_calendar_builder_cases.dart b/mobile/test/features/channels/timeline_calendar_builder_cases.dart
new file mode 100644
index 0000000000..60dfe457a0
--- /dev/null
+++ b/mobile/test/features/channels/timeline_calendar_builder_cases.dart
@@ -0,0 +1,231 @@
+part of 'channel_detail_page_test.dart';
+
+void timelineCalendarBuilderTests() {
+  testWidgets('timeline builders use cached boundaries for every row ending', (
+    tester,
+  ) async {
+    final conversions = <Map<int, int>>[];
+    TimelineCalendarDays createCalendar() {
+      final counts = <int, int>{};
+      conversions.add(counts);
+      return TimelineCalendarDays.withConverter((timestamp) {
+        counts.update(timestamp, (count) => count + 1, ifAbsent: () => 1);
+        // Distinct dates make a stray uncached comparison observable in the
+        // rendered row spacing even though these timestamps share a real day.
+        return DateTime(2030, 1, timestamp);
+      });
+    }
+
+    final root = _textMsg(
+      id: threadIdThreadDashRoot,
+      pubkey: 'alice',
+      content: 'Calendar root',
+      createdAt: 1000,
+    );
+    final replies = [
+      for (final timestamp in [1060, 1120])
+        _textMsg(
+          id: 'calendar-reply-$timestamp',
+          pubkey: 'bob',
+          content: 'Calendar reply $timestamp',
+          createdAt: timestamp,
+          extraTags: const [
+            ['e', threadIdThreadDashRoot, '', 'reply'],
+          ],
+        ),
+    ];
+    await tester.pumpWidget(
+      _buildTestable(
+        calendarDaysFactory: createCalendar,
+        messages: [
+          root,
+          _systemMsg(
+            id: 'calendar-system',
+            createdAt: 1060,
+            payload: {
+              'type': 'member_joined',
+              'actor': 'alice',
+              'target': 'alice',
+            },
+          ),
+          _textMsg(
+            id: 'calendar-last',
+            pubkey: 'bob',
+            content: 'Last',
+            createdAt: 1120,
+          ),
+        ],
+        threadReplies: {threadIdThreadDashRoot: replies},
+        users: const {
+          'alice': UserProfile(pubkey: 'alice', displayName: 'Alice'),
+          'bob': UserProfile(pubkey: 'bob', displayName: 'Bob'),
+        },
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    void expectEndSpacing(String rowKey) {
+      final padding = tester.widget<Padding>(
+        find
+            .descendant(
+              of: find.byKey(ValueKey(rowKey)),
+              matching: find.byType(Padding),
+            )
+            .first,
+      );
+      expect(padding.padding.resolve(TextDirection.ltr).bottom, 0);
+    }
+
+    expectEndSpacing('message-row-$threadIdThreadDashRoot');
+    expectEndSpacing('system-message-row-calendar-system');
+    final head = formatTimeline([root]).single;
+    Navigator.of(tester.element(find.byType(ChannelDetailPage))).push(
+      MaterialPageRoute<void>(
+        builder: (_) => ThreadDetailPage(
+          threadHead: head,
+          allMessages: [head],
+          channelId: _channelId,
+          currentPubkey: 'self',
+          isMember: true,
+          isArchived: false,
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expectEndSpacing('thread-message-row-calendar-reply-1060');
+    final usedCaches = conversions.where((counts) => counts.isNotEmpty);
+    expect(usedCaches, isNotEmpty);
+    for (final counts in usedCaches) {
+      expect(counts.values, everyElement(1));
+    }
+  });
+
+  testWidgets('thread shows day dividers when replies cross days', (
+    tester,
+  ) async {
+    final rootCreatedAt =
+        DateTime(2025, 1, 1, 12).toUtc().millisecondsSinceEpoch ~/ 1000;
+    final nextDayCreatedAt =
+        DateTime(2025, 1, 2, 12).toUtc().millisecondsSinceEpoch ~/ 1000;
+    final rootEvent = _textMsg(
+      id: threadIdThreadDashRoot,
+      pubkey: 'alice',
+      content: 'Thread root',
+      createdAt: rootCreatedAt,
+    );
+    final replies = [
+      _textMsg(
+        id: 'reply-same-day',
+        pubkey: 'bob',
+        content: 'Same day',
+        createdAt: rootCreatedAt + 60,
+        extraTags: const [
+          ['e', threadIdThreadDashRoot, '', 'reply'],
+        ],
+      ),
+      _textMsg(
+        id: 'reply-next-day',
+        pubkey: 'bob',
+        content: 'Next day',
+        createdAt: nextDayCreatedAt,
+        extraTags: const [
+          ['e', threadIdThreadDashRoot, '', 'reply'],
+        ],
+      ),
+    ];
+
+    await tester.pumpWidget(
+      _buildTestable(
+        messages: [rootEvent],
+        threadReplies: {threadIdThreadDashRoot: replies},
+        users: {
+          'alice': const UserProfile(pubkey: 'alice', displayName: 'Alice'),
+          'bob': const UserProfile(pubkey: 'bob', displayName: 'Bob'),
+        },
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    final threadHead = formatTimeline([rootEvent]).single;
+    Navigator.of(tester.element(find.byType(ChannelDetailPage))).push(
+      MaterialPageRoute<void>(
+        builder: (_) => ThreadDetailPage(
+          threadHead: threadHead,
+          allMessages: [threadHead],
+          channelId: _channelId,
+          currentPubkey: 'self',
+          isMember: true,
+          isArchived: false,
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+
+    expect(find.byType(DayDivider), findsNWidgets(2));
+    expect(
+      find.descendant(
+        of: find.byType(DayDivider),
+        matching: find.text(formatDayHeading(rootCreatedAt)),
+      ),
+      findsOneWidget,
+    );
+    expect(
+      find.descendant(
+        of: find.byType(DayDivider),
+        matching: find.text(formatDayHeading(nextDayCreatedAt)),
+      ),
+      findsOneWidget,
+    );
+    // The list runs top-down (head first), so tail spacing lives on the list
+    // and reply groups carry none.
+    final threadList = tester.widget<ScrollablePositionedList>(
+      find.byKey(const ValueKey('thread-message-list')),
+    );
+    expect(threadList.reverse, isFalse);
+    final threadComposerDockHeight = tester
+        .getSize(find.byKey(const ValueKey('thread-composer-dock')))
+        .height;
+    expect(threadList.padding!.bottom, Grid.xs + threadComposerDockHeight);
+    final newestThreadGroup = tester.widget<Padding>(
+      find.byKey(const ValueKey('thread-message-group-reply-next-day')),
+    );
+    expect(newestThreadGroup.padding, EdgeInsets.zero);
+
+    // The head sits above its replies rather than jammed against the
+    // composer, matching desktop's thread panel.
+    final headY = tester
+        .getTopLeft(
+          find.byKey(
+            const ValueKey('thread-message-group-$threadIdThreadDashRoot'),
+          ),
+        )
+        .dy;
+    final oldestReplyY = tester
+        .getTopLeft(
+          find.byKey(const ValueKey('thread-message-group-reply-same-day')),
+        )
+        .dy;
+    final newestReplyY = tester
+        .getTopLeft(
+          find.byKey(const ValueKey('thread-message-group-reply-next-day')),
+        )
+        .dy;
+    expect(headY, lessThan(oldestReplyY));
+    expect(oldestReplyY, lessThan(newestReplyY));
+    final threadTimestamp = tester.widget<Text>(
+      find.byKey(
+        const ValueKey('thread-message-timestamp-$threadIdThreadDashRoot'),
+      ),
+    );
+    expect(threadTimestamp.style?.fontSize, 11);
+    expect(
+      find.descendant(
+        of: find.byKey(
+          const ValueKey('thread-message-row-$threadIdThreadDashRoot'),
+        ),
+        matching: find.text('·'),
+      ),
+      findsNothing,
+    );
+  });
+}