feat: refine task and countdown interactions

frontend/src/lib/task-utils.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from 'vitest'
-import { applyMarkdownFormat, buildTaskRecurrencePayload, buildTaskRrule, classifyTaskForToday, defaultTaskDueAt, filterTasks, groupTaskTree, isSameTaskSortTier, mergeReorderedSubset, moveItemWithinScope, parseTaskRecurrence, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
+import { applyMarkdownFormat, buildTaskDueDraft, buildTaskRecurrencePayload, buildTaskRrule, classifyTaskForToday, defaultTaskDueAt, filterTasks, groupTaskTree, isSameTaskSortTier, mergeReorderedSubset, moveItemWithinScope, parseTaskDueDraft, parseTaskRecurrence, parseTaskRrule, renderMarkdown, toDateTimeLocal } from './task-utils'
 
 type SearchTask = {
   id: string
@@ -136,4 +136,23 @@ describe('task utilities', () => {
     process.env.TZ = original
     expect(toDateTimeLocal(null)).toBe('')
   })
+
+  it('parses date-only and timed task deadlines without inventing a time', () => {
+    const original = process.env.TZ
+    process.env.TZ = 'Asia/Shanghai'
+    expect(parseTaskDueDraft('2026-09-05T15:59:00Z', false)).toEqual({ date: '2026-09-05', hasTime: false, time: '12:00' })
+    expect(parseTaskDueDraft('2026-09-05T12:30:00Z', true)).toEqual({ date: '2026-09-05', hasTime: true, time: '20:30' })
+    expect(parseTaskDueDraft(null, true)).toEqual({ date: '', hasTime: false, time: '12:00' })
+    process.env.TZ = original
+  })
+
+  it('builds date-only, timed, toggled and cleared task deadlines in local time', () => {
+    const original = process.env.TZ
+    process.env.TZ = 'Asia/Shanghai'
+    expect(buildTaskDueDraft({ date: '2026-09-05', hasTime: false, time: '08:15' })).toEqual({ due_at: '2026-09-05T15:59:00.000Z', due_has_time: false })
+    expect(buildTaskDueDraft({ date: '2026-09-05', hasTime: true, time: '08:15' })).toEqual({ due_at: '2026-09-05T00:15:00.000Z', due_has_time: true })
+    expect(buildTaskDueDraft({ date: '2026-09-05', hasTime: true, time: '' })).toEqual({ due_at: '2026-09-05T15:59:00.000Z', due_has_time: false })
+    expect(buildTaskDueDraft({ date: '', hasTime: true, time: '08:15' })).toEqual({ due_at: null, due_has_time: false })
+    process.env.TZ = original
+  })
 })

frontend/src/lib/task-utils.ts

@@ -217,6 +217,22 @@ export function defaultTaskDueAt(now = new Date()) {
   return `${year}-${month}-${day}`
 }
 
+export type TaskDueDraft = { date: string; hasTime: boolean; time: string }
+
+export function parseTaskDueDraft(value: string | null | undefined, dueHasTime: boolean): TaskDueDraft {
+  const local = toDateTimeLocal(value)
+  if (!local) return { date: '', hasTime: false, time: '12:00' }
+  const [date, time = '12:00'] = local.split('T')
+  return { date, hasTime: dueHasTime, time: dueHasTime ? time : '12:00' }
+}
+
+export function buildTaskDueDraft(draft: TaskDueDraft) {
+  if (!draft.date) return { due_at: null, due_has_time: false }
+  const hasTime = draft.hasTime && Boolean(draft.time)
+  const local = `${draft.date}T${hasTime ? draft.time : '23:59'}`
+  return { due_at: fromDateTimeLocal(local), due_has_time: hasTime }
+}
+
 export function toDateTimeLocal(value: string | null | undefined) {
   if (!value) return ''
   const date = new Date(value)