diff --git a/app/celery/nightly_tasks.py b/app/celery/nightly_tasks.py
index 92d401f46..54714922f 100644
--- a/app/celery/nightly_tasks.py
+++ b/app/celery/nightly_tasks.py
@@ -117,9 +117,12 @@ def timeout_notifications():
     # so that we can cope with a high volume that need processing. We've changed
     # dao_timeout_notifications to return up to 100K notifications, so this task
     # will operate on up to 500K - normally we only get around 20K.
+    cutoff_time = datetime.utcnow() - timedelta(
+        seconds=current_app.config.get('SENDING_NOTIFICATIONS_TIMEOUT_PERIOD')
+    )
+
     for _ in range(0, 5):
-        notifications = \
-            dao_timeout_notifications(current_app.config.get('SENDING_NOTIFICATIONS_TIMEOUT_PERIOD'))
+        notifications = dao_timeout_notifications(cutoff_time)
 
         for notification in notifications:
             check_and_queue_callback_task(notification)
diff --git a/app/dao/notifications_dao.py b/app/dao/notifications_dao.py
index 15daca241..acfa286e3 100644
--- a/app/dao/notifications_dao.py
+++ b/app/dao/notifications_dao.py
@@ -489,11 +489,11 @@ def dao_delete_notifications_by_id(notification_id):
     ).delete(synchronize_session='fetch')
 
 
-def dao_timeout_notifications(timeout_period_in_seconds):
+def dao_timeout_notifications(cutoff_time):
     """
-    Set email and SMS notifications (only) to "temporary-failure" status.
+    Set email and SMS notifications (only) to "temporary-failure" status
+    if they're still sending from before the specified cutoff_time.
     """
-    timeout_start = datetime.utcnow() - timedelta(seconds=timeout_period_in_seconds)
     updated_at = datetime.utcnow()
     current_statuses = [NOTIFICATION_SENDING, NOTIFICATION_PENDING]
     new_status = NOTIFICATION_TEMPORARY_FAILURE
@@ -503,13 +503,13 @@ def dao_timeout_notifications(timeout_period_in_seconds):
     # - we believe this is a OOM error but there are no logs. From
     # experimentation we've found we can safely process up to 100K.
     notifications = Notification.query.filter(
-        Notification.created_at < timeout_start,
+        Notification.created_at < cutoff_time,
         Notification.status.in_(current_statuses),
         Notification.notification_type.in_([SMS_TYPE, EMAIL_TYPE])
     ).limit(100000).all()
 
     Notification.query.filter(
-        Notification.created_at < timeout_start,
+        Notification.created_at < cutoff_time,
         Notification.status.in_(current_statuses),
         Notification.notification_type.in_([SMS_TYPE, EMAIL_TYPE]),
         Notification.id.in_([n.id for n in notifications]),
diff --git a/tests/app/celery/test_nightly_tasks.py b/tests/app/celery/test_nightly_tasks.py
index 7a6167aa2..847a27ce9 100644
--- a/tests/app/celery/test_nightly_tasks.py
+++ b/tests/app/celery/test_nightly_tasks.py
@@ -161,17 +161,14 @@ def test_delete_letter_notifications_older_than_retention_calls_child_task(notif
     mocked.assert_called_once_with('letter')
 
 
+@freeze_time("2021-12-13T10:00")
 def test_timeout_notifications(mocker, sample_notification):
     mock_update = mocker.patch('app.celery.nightly_tasks.check_and_queue_callback_task')
     mock_dao = mocker.patch('app.celery.nightly_tasks.dao_timeout_notifications')
     mock_dao.return_value = [sample_notification]
 
     timeout_notifications()
-
-    mock_dao.assert_called_once_with(
-        current_app.config.get('SENDING_NOTIFICATIONS_TIMEOUT_PERIOD')
-    )
-
+    mock_dao.assert_called_once_with(datetime.fromisoformat('2021-12-10T10:00'))
     mock_update.assert_called_once_with(sample_notification)
 
 
diff --git a/tests/app/dao/notification_dao/test_notification_dao.py b/tests/app/dao/notification_dao/test_notification_dao.py
index 8e5f495e2..e09bb26d8 100644
--- a/tests/app/dao/notification_dao/test_notification_dao.py
+++ b/tests/app/dao/notification_dao/test_notification_dao.py
@@ -671,7 +671,7 @@ def test_dao_timeout_notifications(sample_template):
         pending = create_notification(sample_template, status='pending')
         delivered = create_notification(sample_template, status='delivered')
 
-    temporary_failure_notifications = dao_timeout_notifications(1)
+    temporary_failure_notifications = dao_timeout_notifications(datetime.utcnow())
 
     assert len(temporary_failure_notifications) == 2
     assert Notification.query.get(created.id).status == 'created'
@@ -687,7 +687,7 @@ def test_dao_timeout_notifications_only_updates_for_older_notifications(sample_t
         pending = create_notification(sample_template, status='pending')
         delivered = create_notification(sample_template, status='delivered')
 
-    temporary_failure_notifications = dao_timeout_notifications(1)
+    temporary_failure_notifications = dao_timeout_notifications(datetime.utcnow())
 
     assert len(temporary_failure_notifications) == 0
     assert Notification.query.get(created.id).status == 'created'
@@ -703,7 +703,7 @@ def test_dao_timeout_notifications_doesnt_affect_letters(sample_letter_template)
         pending = create_notification(sample_letter_template, status='pending')
         delivered = create_notification(sample_letter_template, status='delivered')
 
-    temporary_failure_notifications = dao_timeout_notifications(1)
+    temporary_failure_notifications = dao_timeout_notifications(datetime.utcnow())
 
     assert len(temporary_failure_notifications) == 0
     assert Notification.query.get(created.id).status == 'created'