diff --git a/backend/app/chat/log.py b/backend/app/chat/log.py
index 7a104110..b00e99f2 100644
--- a/backend/app/chat/log.py
+++ b/backend/app/chat/log.py
@@ -197,12 +197,13 @@ async def stream(turn_id: str, after: int, *, active: Callable[[], Awaitable[str
                 yield PING
 
 
-async def purge(now: datetime, *, retain_days: float = RETAIN_DAYS, limit: int = PURGE_BATCH) -> list[str]:
+async def purge(now: datetime | None = None, *, retain_days: float = RETAIN_DAYS, limit: int = PURGE_BATCH) -> list[str]:
     """结束超过 retain_days 天的轮次整轮删事件日志（见模块说明），交回清了哪几轮。任务行、conversation_turns 那一行不动。
     结束的时刻按任务的 finished_at，没记的按 updated_at。"""
+    from app.db.base import utcnow
     from app.db.models_chat import ChatTurnEvent, ChatTurnJob
 
-    cutoff = now - timedelta(days=retain_days)
+    cutoff = (now or utcnow()) - timedelta(days=retain_days)
     async with SessionLocal() as session:
         ended = select(ChatTurnJob.id).where(ChatTurnJob.status.in_(FINISHED),
                                              func.coalesce(ChatTurnJob.finished_at, ChatTurnJob.updated_at) < cutoff)
@@ -218,8 +219,8 @@ async def purge(now: datetime, *, retain_days: float = RETAIN_DAYS, limit: int =
 _last_purge = 0.0
 
 
-async def purge_tick(now: datetime) -> list[str]:
-    """调度循环每一拍调（scheduler/loop.process_due）：一小时最多清一次。"""
+async def purge_tick(now: datetime | None = None) -> list[str]:
+    """调度循环每一拍调（scheduler/loop.process_due，那一拍不一定给 now）：一小时最多清一次，不给 now 按当前时间算。"""
     global _last_purge
     if time.monotonic() - _last_purge < PURGE_EVERY_S and _last_purge:
         return []
diff --git a/backend/tests/test_chat_turns.py b/backend/tests/test_chat_turns.py
index f5f851b3..77104fe7 100644
--- a/backend/tests/test_chat_turns.py
+++ b/backend/tests/test_chat_turns.py
@@ -1155,3 +1155,17 @@ async def test_a_downgraded_account_that_can_still_chat_finishes_as_who_they_are
     row = await _row(turn_id)
     assert row.status == "done" and row.answer == "华东 300。"
     assert seen and seen[-1].username == "bob" and seen[-1].role == "user"
+
+
+async def test_the_cleanup_tick_without_a_time_uses_now(monkeypatch):
+    """调度循环那一拍不一定给 now（process_due(None)）：清理按当前时间算，不报错。"""
+    from app.scheduler import loop
+
+    old = await _finished_turn(days_ago=turn_log.RETAIN_DAYS + 2)
+    monkeypatch.setattr(turn_log, "_last_purge", 0.0)
+    assert old in await turn_log.purge_tick()
+    assert await _events(old) == []
+    again = await _finished_turn(days_ago=turn_log.RETAIN_DAYS + 2)
+    monkeypatch.setattr(turn_log, "_last_purge", 0.0)
+    await loop.process_due(None)
+    assert await _events(again) == []