diff --git a/docs/review/execution-timing-io-2026-10-04.md b/docs/review/execution-timing-io-2026-10-04.md
index fc31c4c3..00d91b69 100644
--- a/docs/review/execution-timing-io-2026-10-04.md
+++ b/docs/review/execution-timing-io-2026-10-04.md
@@ -14,6 +14,13 @@ requests remain paused during reconnection. Startup preserves closed intervals,
 discards unclosed intervals, and marks them incomplete; old history is not
 backfilled by scanning messages or events.
 
+An incomplete zero duration means no known timed interval, not a measured zero.
+Both conversation totals and task cards display "unavailable" in this case.
+Positive partial durations retain the incomplete warning; complete zero durations
+still display zero. The upgrade regression verifies that a new completed reply
+in an old conversation retains its accumulated milliseconds after reopening the
+database. This presentation fix does not restore pre-upgrade historical totals.
+
 Schema 59 stores timing rows and adds a covering index for conversation
 summaries. The old historical aggregation, its caches, dedicated reader/worker,
 worker build entries, and five-second statistics polling have been removed.
diff --git a/src/main/assistant/assistant-execution-stats.test.ts b/src/main/assistant/assistant-execution-stats.test.ts
index 3b05aee6..2b34f11f 100644
--- a/src/main/assistant/assistant-execution-stats.test.ts
+++ b/src/main/assistant/assistant-execution-stats.test.ts
@@ -240,6 +240,18 @@ describe('persisted running-state duration', () => {
     expect(raw().prepare('PRAGMA index_info(messages_summary_idx)').all().map(row => row.name))
       .toEqual(['conversation_id', 'sequence', 'role', 'created_at'])
     expect(raw().prepare('PRAGMA integrity_check').get()).toEqual({ integrity_check: 'ok' })
+    const replyId = task()
+    database.startExecutionTiming(replyId)
+    now += 7000
+    database.updateTaskStatus(replyId, 'completed')
+    database.endExecutionTiming(replyId)
+    expect(stats()).toMatchObject({ durationMs: 7000, runningCount: 0, incomplete: true })
+    database.close()
+    now += 86_400_000
+    database.initialize(directory)
+    expect(stats()).toMatchObject({ durationMs: 7000, runningCount: 0, incomplete: true })
+    expect(raw().prepare('SELECT duration_ms FROM execution_timing WHERE task_id = ?').get(replyId))
+      .toEqual({ duration_ms: 7000 })
   })
 
   it('deletes timing with its conversation and notifies snapshot subscribers', async () => {
diff --git a/src/renderer/src/RightAssistantSidebar.resize.test.tsx b/src/renderer/src/RightAssistantSidebar.resize.test.tsx
index 2dd33693..9692c960 100644
--- a/src/renderer/src/RightAssistantSidebar.resize.test.tsx
+++ b/src/renderer/src/RightAssistantSidebar.resize.test.tsx
@@ -454,6 +454,20 @@ describe('RightAssistantSidebar resizing', () => {
     expect(screen.getByText('Project task a').closest('button')).toHaveAttribute('aria-current', 'true')
     expect(screen.getByText('Project task b').closest('article')).not.toHaveTextContent('任务累计用时')
 
+    view.rerender(sidebarElement({ ...props,
+      conversationStats: { ...props.conversationStats, replyDurationMs: 0 },
+      taskDurations: new Map([['a', { durationMs: 0, incomplete: true }]])
+    }))
+    expect(stats.getByText('暂无统计')).toBeVisible()
+    expect(stats.queryByText('00:00:00')).not.toBeInTheDocument()
+    expect(screen.getByText('任务累计用时: 暂无统计 (部分记录)')).toBeVisible()
+    view.rerender(sidebarElement({ ...props,
+      conversationStats: { ...props.conversationStats, replyDurationMs: 500 },
+      taskDurations: new Map([['a', { durationMs: 500, incomplete: true }]])
+    }))
+    expect(stats.getByText('00:00:00')).toBeVisible()
+    expect(screen.getByText('任务累计用时: 00:00:00 (部分记录)')).toBeVisible()
+
     view.rerender(sidebarElement({ ...props, activeConversationId: 'b' }))
     expect(stats.queryByText('Conversation A')).not.toBeInTheDocument()
     expect(stats.getAllByText('暂无统计')).toHaveLength(2)
diff --git a/src/renderer/src/RightAssistantSidebar.tsx b/src/renderer/src/RightAssistantSidebar.tsx
index 7eb066d2..585b1d8d 100644
--- a/src/renderer/src/RightAssistantSidebar.tsx
+++ b/src/renderer/src/RightAssistantSidebar.tsx
@@ -838,7 +838,8 @@ function RightAssistantSidebarView({
   const currentStats = activeConversationId && conversationStats?.conversationId === activeConversationId
     ? conversationStats
     : undefined
-  const formatDuration = (durationMs: number): string => {
+  const formatDuration = (durationMs: number, incomplete = false): string => {
+    if (incomplete && durationMs === 0) return t('sidebar.tasks.stats.unavailable')
     const seconds = Math.floor(Math.max(0, durationMs) / 1000)
     return `${Math.floor(seconds / 3600).toString().padStart(2, '0')}:${Math.floor(seconds / 60 % 60).toString().padStart(2, '0')}:${(seconds % 60).toString().padStart(2, '0')}`
   }
@@ -2105,7 +2106,7 @@ function RightAssistantSidebarView({
                 ? currentStats?.title ?? conversationTitles.get(activeConversationId) ?? t('sidebar.tasks.stats.currentConversation')
                 : t('sidebar.tasks.stats.noConversation')}</h3>
               <dl className="task-center__stat-grid">
-                <div><dt>{t('sidebar.tasks.stats.replyDuration')}</dt><dd>{currentStats ? formatDuration(currentStats.replyDurationMs) : t('sidebar.tasks.stats.unavailable')}</dd></div>
+                <div><dt>{t('sidebar.tasks.stats.replyDuration')}</dt><dd>{currentStats ? formatDuration(currentStats.replyDurationMs, currentStats.incomplete) : t('sidebar.tasks.stats.unavailable')}</dd></div>
                 <div><dt>{t('sidebar.tasks.stats.messages')}</dt><dd>{currentStats ? currentStats.messageCount.toLocaleString(locale) : t('sidebar.tasks.stats.unavailable')}</dd></div>
               </dl>
               {currentStats?.incomplete && <p className="task-center__incomplete">{t('sidebar.tasks.stats.incomplete')}</p>}
@@ -2253,7 +2254,7 @@ function RightAssistantSidebarView({
                       </span>
                       <span>{t(`task.status.${task.status}`)}</span>
                       {duration && <span title={t('sidebar.tasks.stats.taskDuration')}>
-                        {t('sidebar.tasks.stats.taskDuration')}: {formatDuration(duration.durationMs)}
+                        {t('sidebar.tasks.stats.taskDuration')}: {formatDuration(duration.durationMs, duration.incomplete)}
                         {duration.incomplete && ` (${t('sidebar.tasks.stats.partial')})`}
                       </span>}
                     </div>