diff --git a/apps/web/app/overtime/page.tsx b/apps/web/app/overtime/page.tsx
index 9722ad7..124ae0b 100644
--- a/apps/web/app/overtime/page.tsx
+++ b/apps/web/app/overtime/page.tsx
@@ -15,6 +15,7 @@ import { MonthPicker } from '@/components/MonthPicker';
 import { FilterSelect } from '@/components/FilterSelect';
 import { FieldError } from '@/components/FieldError';
 import { RouteGuard } from '@/components/auth/Guard';
+import { WorkDateTimePicker } from '@/components/WorkDateTimePicker';
 
 export default function OvertimePage() {
   return (
@@ -287,11 +288,22 @@ function OvertimeModal({ defaultPerson, products, projects, versions, reasons, r
           <div className="grid grid-cols-2 gap-3">
             <div>
               <label className="text-[12px] font-medium text-[var(--ink-soft)] mb-1 block">开始时间 *</label>
-              <input type="datetime-local" value={startTime} onChange={(e) => { setStartTime(e.target.value); setEndTimeError(''); }} required className="h-9 w-full rounded-lg border border-[var(--line)] bg-[var(--bg-card)] px-3 text-[13px] focus:border-[var(--accent)] focus:outline-none" />
+              <WorkDateTimePicker
+                value={startTime}
+                onChange={(next) => { setStartTime(next); setEndTimeError(''); }}
+                placeholder="选择加班开始时间"
+                defaultHour={19}
+              />
             </div>
             <div>
               <label className="text-[12px] font-medium text-[var(--ink-soft)] mb-1 block">结束时间 *</label>
-              <input type="datetime-local" value={endTime} onChange={(e) => { setEndTime(e.target.value); setEndTimeError(''); }} required className="h-9 w-full rounded-lg border border-[var(--line)] bg-[var(--bg-card)] px-3 text-[13px] focus:border-[var(--accent)] focus:outline-none" />
+              <WorkDateTimePicker
+                value={endTime}
+                onChange={(next) => { setEndTime(next); setEndTimeError(''); }}
+                placeholder="选择加班结束时间"
+                defaultHour={21}
+                popoverAlign="right"
+              />
               <FieldError message={endTimeError} />
             </div>
           </div>
diff --git a/apps/web/lib/overtime.ts b/apps/web/lib/overtime.ts
index 8cc200d..ce13719 100644
--- a/apps/web/lib/overtime.ts
+++ b/apps/web/lib/overtime.ts
@@ -39,9 +39,69 @@ export const OVERTIME_REASON_LABEL: Record<string, string> = {
   'reason-11': '返工修改',
 };
 
-export function calcDuration(start: string, end: string): number {
-  const s = new Date(start).getTime();
-  const e = new Date(end).getTime();
-  if (isNaN(s) || isNaN(e) || e <= s) return 0;
-  return Math.round(((e - s) / (1000 * 60 * 60)) * 10) / 10;
+const MS_PER_HOUR = 60 * 60 * 1000;
+const STANDARD_START_HOUR = 9;
+const LUNCH_START_HOUR = 12;
+const LUNCH_END_HOUR = 13;
+const STANDARD_END_HOUR = 18;
+
+function startOfDay(date: Date): Date {
+  const day = new Date(date);
+  day.setHours(0, 0, 0, 0);
+  return day;
+}
+
+function dayBound(day: Date, hour: number): Date {
+  const result = new Date(day);
+  result.setHours(hour, 0, 0, 0);
+  return result;
+}
+
+function isSameLocalDay(a: Date, b: Date): boolean {
+  return a.getFullYear() === b.getFullYear()
+    && a.getMonth() === b.getMonth()
+    && a.getDate() === b.getDate();
+}
+
+function overlapHours(start: Date, end: Date, segmentStart: Date, segmentEnd: Date): number {
+  const overlapStart = Math.max(start.getTime(), segmentStart.getTime());
+  const overlapEnd = Math.min(end.getTime(), segmentEnd.getTime());
+  return Math.max(0, overlapEnd - overlapStart) / MS_PER_HOUR;
+}
+
+function calcProjectDayHours(start: Date, end: Date): number {
+  if (end.getTime() <= start.getTime()) return 0;
+  const day = startOfDay(start);
+  const gross = (end.getTime() - start.getTime()) / MS_PER_HOUR;
+  const lunch = overlapHours(
+    start,
+    end,
+    dayBound(day, LUNCH_START_HOUR),
+    dayBound(day, LUNCH_END_HOUR),
+  );
+  return Math.max(0, gross - lunch);
+}
+
+export function calcDuration(start: string, end: string): number {
+  const startDate = new Date(start);
+  const endDate = new Date(end);
+  const s = startDate.getTime();
+  const e = endDate.getTime();
+  if (isNaN(s) || isNaN(e) || e <= s) return 0;
+
+  let total = 0;
+  const cursor = startOfDay(startDate);
+  const lastDay = startOfDay(endDate);
+
+  while (cursor.getTime() <= lastDay.getTime()) {
+    const isFirst = isSameLocalDay(cursor, startDate);
+    const isLast = isSameLocalDay(cursor, endDate);
+    const dayStart = isFirst ? new Date(startDate) : dayBound(cursor, STANDARD_START_HOUR);
+    const dayEnd = isLast ? new Date(endDate) : dayBound(cursor, STANDARD_END_HOUR);
+
+    total += calcProjectDayHours(dayStart, dayEnd);
+    cursor.setDate(cursor.getDate() + 1);
+  }
+
+  return Math.round(total * 10) / 10;
 }
diff --git a/apps/web/lib/work-hours.test.ts b/apps/web/lib/work-hours.test.ts
index 7b07830..93a501b 100644
--- a/apps/web/lib/work-hours.test.ts
+++ b/apps/web/lib/work-hours.test.ts
@@ -17,8 +17,13 @@ test('calcActualElapsedHours uses work calendar time with a half-hour minimum',
   assert.equal(calcActualElapsedHours('2026-06-22T10:00:00', '2026-06-22T09:00:00'), 0);
 });
 
-test('overtime duration still counts holiday time directly', () => {
-  assert.equal(calcDuration('2026-10-01T09:00:00', '2026-10-01T18:00:00'), 9);
+test('overtime duration uses project day rhythm instead of continuous clock time', () => {
+  assert.equal(calcDuration('2026-07-01T09:00:00', '2026-07-02T18:00:00'), 16);
+  assert.equal(calcDuration('2026-07-01T09:00:00', '2026-07-02T20:00:00'), 18);
+});
+
+test('overtime duration counts holidays as selectable work days without skipping them', () => {
+  assert.equal(calcDuration('2026-10-01T09:00:00', '2026-10-01T18:00:00'), 8);
 });
 
 test('formatActualDuration converts hours to natural days', () => {
diff --git a/docs/decisions.md b/docs/decisions.md
index 8c97a16..8840f02 100644
--- a/docs/decisions.md
+++ b/docs/decisions.md
@@ -364,6 +364,6 @@
 - `calcWorkHours` 和正常任务的 `calcActualElapsedHours` 都按中国工作日历和工作时段计算。
 - 法定节假日、周末不计入正常工时；调休上班日计入正常工时。
 - AI 预估工时是工作量建议，不按日期跳过；执行预估只有从计划起止时间自动推导时才走工作日历。
-- 加班记录不受工作日历过滤，按加班单填写的开始/结束时间直接计入额外投入。
+- 加班记录不受工作日历过滤，但不按自然小时连续打卡计算；使用项目管理口径：每个自然日默认 9:00-12:00、13:00-18:00 计 8h，中间完整日期按 8h，首尾日期按填写的开始/结束时间裁剪，结束晚于 18:00 的当天额外计入超出时长。
 
-**理由**：正常任务耗时回答“工作时间里实际投入了多少”，加班记录回答“非正常工作时间额外投入了多少”，两条口径分开才能避免休息日被重复或误算。
+**理由**：正常任务耗时回答“工作时间里实际投入了多少”，加班记录回答“额外投入覆盖了多少项目工作量”。项目管理不做打卡，跨天记录不能把夜间空档算成工时；但节假日和周末仍允许记录，不受中国工作日历过滤。
diff --git a/docs/workflow.md b/docs/workflow.md
index 08bb0ff..09b0342 100644
--- a/docs/workflow.md
+++ b/docs/workflow.md
@@ -239,4 +239,5 @@ Implementation convention:
 - 正常任务耗时使用 `calcWorkHours` / `calcActualElapsedHours`，按中国工作日历和 9:00-12:00、13:00-18:00 工作时段计算。
 - 法定节假日和周末不计入正常任务耗时；调休上班日计入正常任务耗时。
 - AI 预估工时和手填执行预估工时本身是小时数，不再按日期过滤；只有从计划起止时间自动推导的执行预估会按工作日历计算。
-- 加班记录使用 `overtime.calcDuration`，按加班单开始/结束时间直接计算，不受节假日过滤。
+- 加班记录使用 `overtime.calcDuration`，不受节假日过滤，但按项目管理口径计算：每个自然日默认最多 8h，跨天中间日期按 8h，结束晚于 18:00 的当天额外计入超出时长，避免把夜间空档当作打卡工时。
+- 新建加班记录也使用工作日历日期选择器；节假日和周末可选，只提示不阻止。