From 6571cfc8eb89a4bfdce355e710a8f7816a123eb7 Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Wed, 24 Jun 2026 15:41:04 -0700
Subject: [PATCH 01/10] feat(nimbus): add weekly Celery task to update holdback
 enrollment period and trigger Jetstream rerun

---
 experimenter/experimenter/jetstream/tasks.py | 49 ++++++++++++++++++++
 experimenter/experimenter/settings.py        |  4 ++
 2 files changed, 53 insertions(+)

diff --git a/experimenter/experimenter/jetstream/tasks.py b/experimenter/experimenter/jetstream/tasks.py
index 2dc1b0396..7615e05ff 100644
--- a/experimenter/experimenter/jetstream/tasks.py
+++ b/experimenter/experimenter/jetstream/tasks.py
@@ -196,3 +196,52 @@ def fetch_monitoring_data():
         metrics.incr("fetch_monitoring_data.failed")
         logger.exception(f"Fatal error in fetch_monitoring_data task: {e}")
         raise
+
+
+@app.task
+@metrics.timer_decorator("update_holdback_enrollment_period")
+def update_holdback_enrollment_period():
+    metrics.incr("update_holdback_enrollment_period.started")
+    try:
+        today = timezone.now().date()
+        now = timezone.now()
+
+        experiments = NimbusExperiment.objects.filter(
+            is_holdback=True,
+            status=NimbusExperiment.Status.LIVE,
+        ).exclude(_start_date=None)
+
+        updated_count = 0
+        for experiment in experiments:
+            enrollment_end = experiment.actual_enrollment_end_date
+            if enrollment_end is None or today <= enrollment_end:
+                continue
+
+            days_since_end = (today - enrollment_end).days
+            weeks_elapsed = max(1, days_since_end // 7)
+            new_enrollment_period = min(
+                experiment.proposed_enrollment + (weeks_elapsed * 7),
+                experiment.proposed_duration,
+            )
+
+            experiment.proposed_enrollment = new_enrollment_period
+            experiment.do_rerun = True
+            experiment.do_rerun_timestamp = now
+            experiment.save(
+                update_fields=[
+                    "proposed_enrollment",
+                    "do_rerun",
+                    "do_rerun_timestamp",
+                ]
+            )
+            updated_count += 1
+
+        logger.info(
+            f"update_holdback_enrollment_period: updated {updated_count} experiments"
+        )
+        metrics.incr("update_holdback_enrollment_period.completed")
+
+    except Exception as e:
+        metrics.incr("update_holdback_enrollment_period.failed")
+        logger.exception(f"Fatal error in update_holdback_enrollment_period: {e}")
+        raise
diff --git a/experimenter/experimenter/settings.py b/experimenter/experimenter/settings.py
index 9c90f8e08..03d6b03f9 100644
--- a/experimenter/experimenter/settings.py
+++ b/experimenter/experimenter/settings.py
@@ -421,6 +421,10 @@
         "task": "experimenter.experiments.tasks.warm_api_caches",
         "schedule": config("API_CACHE_WARMING_INTERVAL", default=3600, cast=int),
     },
+    "update_holdback_enrollment_period": {
+        "task": "experimenter.jetstream.tasks.update_holdback_enrollment_period",
+        "schedule": crontab(minute=0, hour=7, day_of_week=1),
+    },
 }
 CELERY_TASK_ROUTES = {
     "experimenter.kinto.tasks.*": {"queue": "remote_settings"},

From 0729b504bb3c2920f4d3c7a5d8dc6708ae9a3eb8 Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Wed, 24 Jun 2026 15:49:08 -0700
Subject: [PATCH 02/10] test(nimbus): add tests for
 update_holdback_enrollment_period task

---
 .../jetstream/tests/test_tasks.py             | 79 +++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/experimenter/experimenter/jetstream/tests/test_tasks.py b/experimenter/experimenter/jetstream/tests/test_tasks.py
index a7a950462..a32c06a4e 100644
--- a/experimenter/experimenter/jetstream/tests/test_tasks.py
+++ b/experimenter/experimenter/jetstream/tests/test_tasks.py
@@ -3949,3 +3949,82 @@ def test_result_is_cached(self):
             get_featmon_slugs()
             get_featmon_slugs()
             mock_read.assert_called_once()
+
+
+class TestUpdateHoldbackEnrollmentPeriod(TestCase):
+    def _make_holdback(self, start_date, enrollment_days, duration_days, end_date=None):
+        experiment = NimbusExperimentFactory.create_with_lifecycle(
+            NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
+            is_holdback=True,
+            _start_date=start_date,
+            proposed_enrollment=enrollment_days,
+            proposed_duration=duration_days,
+        )
+        if end_date:
+            experiment._computed_end_date = end_date
+            experiment.save(update_fields=["_computed_end_date"])
+        return experiment
+
+    def test_updates_enrollment_period_and_sets_do_rerun(self):
+        today = datetime.date.today()
+        start = today - datetime.timedelta(days=50)
+        # enrollment ended 22 days ago (14-day enrollment period started at start)
+        experiment = NimbusExperimentFactory.create_with_lifecycle(
+            NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
+            is_holdback=True,
+            _start_date=start,
+            proposed_enrollment=14,
+            proposed_duration=84,
+        )
+        tasks.update_holdback_enrollment_period()
+        experiment.refresh_from_db()
+
+        # weeks_elapsed = 22 // 7 = 3 → 14 + 21 = 35
+        self.assertEqual(experiment.proposed_enrollment, 35)
+        self.assertTrue(experiment.do_rerun)
+        self.assertIsNotNone(experiment.do_rerun_timestamp)
+
+    def test_skips_experiment_still_in_enrollment(self):
+        today = datetime.date.today()
+        experiment = NimbusExperimentFactory.create_with_lifecycle(
+            NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
+            is_holdback=True,
+            _start_date=today - datetime.timedelta(days=5),
+            proposed_enrollment=14,
+            proposed_duration=84,
+        )
+        original_enrollment = experiment.proposed_enrollment
+        tasks.update_holdback_enrollment_period()
+        experiment.refresh_from_db()
+
+        self.assertEqual(experiment.proposed_enrollment, original_enrollment)
+        self.assertFalse(experiment.do_rerun)
+
+    def test_skips_non_holdback_experiments(self):
+        today = datetime.date.today()
+        experiment = NimbusExperimentFactory.create_with_lifecycle(
+            NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
+            is_holdback=False,
+            _start_date=today - datetime.timedelta(days=50),
+            proposed_enrollment=14,
+            proposed_duration=84,
+        )
+        tasks.update_holdback_enrollment_period()
+        experiment.refresh_from_db()
+
+        self.assertFalse(experiment.do_rerun)
+
+    def test_caps_enrollment_at_proposed_duration(self):
+        today = datetime.date.today()
+        experiment = NimbusExperimentFactory.create_with_lifecycle(
+            NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
+            is_holdback=True,
+            _start_date=today - datetime.timedelta(days=200),
+            proposed_enrollment=14,
+            proposed_duration=84,
+        )
+        tasks.update_holdback_enrollment_period()
+        experiment.refresh_from_db()
+
+        self.assertEqual(experiment.proposed_enrollment, 84)
+        self.assertTrue(experiment.do_rerun)

From e85d07631867e4e94f51b88ef9ffddac4b91ea66 Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Thu, 25 Jun 2026 08:48:07 -0700
Subject: [PATCH 03/10] fix(nimbus): use computed_enrollment_end_date in
 holdback task and fix test expectations
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Holdback experiments never pause enrollment so actual_enrollment_end_date
is always None, causing the task to skip every experiment. Switch to
computed_enrollment_end_date (start_date + proposed_enrollment) which
correctly reflects when the initial enrollment window expires.

Update test comment and expected value to match the corrected calculation
(36 days since end → weeks_elapsed=5 → 14+35=49).
---
 experimenter/experimenter/jetstream/tasks.py            | 2 +-
 experimenter/experimenter/jetstream/tests/test_tasks.py | 6 +++---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/experimenter/experimenter/jetstream/tasks.py b/experimenter/experimenter/jetstream/tasks.py
index 7615e05ff..5078a4a7b 100644
--- a/experimenter/experimenter/jetstream/tasks.py
+++ b/experimenter/experimenter/jetstream/tasks.py
@@ -213,7 +213,7 @@ def update_holdback_enrollment_period():
 
         updated_count = 0
         for experiment in experiments:
-            enrollment_end = experiment.actual_enrollment_end_date
+            enrollment_end = experiment.computed_enrollment_end_date
             if enrollment_end is None or today <= enrollment_end:
                 continue
 
diff --git a/experimenter/experimenter/jetstream/tests/test_tasks.py b/experimenter/experimenter/jetstream/tests/test_tasks.py
index a32c06a4e..635bc6c90 100644
--- a/experimenter/experimenter/jetstream/tests/test_tasks.py
+++ b/experimenter/experimenter/jetstream/tests/test_tasks.py
@@ -3968,7 +3968,7 @@ def _make_holdback(self, start_date, enrollment_days, duration_days, end_date=No
     def test_updates_enrollment_period_and_sets_do_rerun(self):
         today = datetime.date.today()
         start = today - datetime.timedelta(days=50)
-        # enrollment ended 22 days ago (14-day enrollment period started at start)
+        # enrollment ended 36 days ago (14-day enrollment period started at start)
         experiment = NimbusExperimentFactory.create_with_lifecycle(
             NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
             is_holdback=True,
@@ -3979,8 +3979,8 @@ def test_updates_enrollment_period_and_sets_do_rerun(self):
         tasks.update_holdback_enrollment_period()
         experiment.refresh_from_db()
 
-        # weeks_elapsed = 22 // 7 = 3 → 14 + 21 = 35
-        self.assertEqual(experiment.proposed_enrollment, 35)
+        # weeks_elapsed = 36 // 7 = 5 → 14 + 35 = 49
+        self.assertEqual(experiment.proposed_enrollment, 49)
         self.assertTrue(experiment.do_rerun)
         self.assertIsNotNone(experiment.do_rerun_timestamp)
 

From 932752b4fab55615fde1dacc85a18f34fa5d6d7b Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Mon, 29 Jun 2026 09:58:25 -0700
Subject: [PATCH 04/10] test(nimbus): add error path coverage for
 update_holdback_enrollment_period

---
 experimenter/experimenter/jetstream/tests/test_tasks.py | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/experimenter/experimenter/jetstream/tests/test_tasks.py b/experimenter/experimenter/jetstream/tests/test_tasks.py
index 635bc6c90..a4a689004 100644
--- a/experimenter/experimenter/jetstream/tests/test_tasks.py
+++ b/experimenter/experimenter/jetstream/tests/test_tasks.py
@@ -4028,3 +4028,10 @@ def test_caps_enrollment_at_proposed_duration(self):
 
         self.assertEqual(experiment.proposed_enrollment, 84)
         self.assertTrue(experiment.do_rerun)
+
+    def test_raises_on_unexpected_error(self):
+        with patch(
+            "experimenter.jetstream.tasks.NimbusExperiment.objects.filter",
+            side_effect=Exception("db error"),
+        ), self.assertRaises(Exception, msg="db error"):
+            tasks.update_holdback_enrollment_period()

From d286993f53beb6a44bf3e293b534f869d7945957 Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Mon, 29 Jun 2026 11:02:14 -0700
Subject: [PATCH 05/10] style(nimbus): reformat test_raises_on_unexpected_error
 with parenthesized context managers

---
 .../experimenter/jetstream/tests/test_tasks.py        | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/experimenter/experimenter/jetstream/tests/test_tasks.py b/experimenter/experimenter/jetstream/tests/test_tasks.py
index a4a689004..d92a08e5f 100644
--- a/experimenter/experimenter/jetstream/tests/test_tasks.py
+++ b/experimenter/experimenter/jetstream/tests/test_tasks.py
@@ -4030,8 +4030,11 @@ def test_caps_enrollment_at_proposed_duration(self):
         self.assertTrue(experiment.do_rerun)
 
     def test_raises_on_unexpected_error(self):
-        with patch(
-            "experimenter.jetstream.tasks.NimbusExperiment.objects.filter",
-            side_effect=Exception("db error"),
-        ), self.assertRaises(Exception, msg="db error"):
+        with (
+            patch(
+                "experimenter.jetstream.tasks.NimbusExperiment.objects.filter",
+                side_effect=Exception("db error"),
+            ),
+            self.assertRaises(Exception, msg="db error"),
+        ):
             tasks.update_holdback_enrollment_period()

From 3f31ba2707e9f52993f51197c902980c0c163ed8 Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Tue, 30 Jun 2026 15:44:01 -0700
Subject: [PATCH 06/10] fix(nimbus): rework holdback task and serializer based
 on review feedback
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

- Run daily instead of weekly (holdbacks can launch any day)
- Use today - 21 days as rolling enrollment_end (observation period)
- Skip experiments with end_date set (actually ended holdbacks)
- Update _enrollment_end_date instead of proposed_enrollment
- Add changelog entry per updated experiment
- Compute proposedEnrollment, enrollmentEndDate, endDate on-the-fly
  in v8 serializer for active holdbacks — real fields stay untouched
- Add HOLDBACK_ENROLLMENT_UPDATED changelog message constant
---
 .../experiments/api/v8/serializers.py         | 27 ++++++++++--
 .../experimenter/experiments/models.py        |  1 +
 experimenter/experimenter/jetstream/tasks.py  | 24 ++++++-----
 .../jetstream/tests/test_tasks.py             | 43 +++++++------------
 experimenter/experimenter/settings.py         |  2 +-
 5 files changed, 54 insertions(+), 43 deletions(-)

diff --git a/experimenter/experimenter/experiments/api/v8/serializers.py b/experimenter/experimenter/experiments/api/v8/serializers.py
index a15ae6f19..ad8a63c5f 100644
--- a/experimenter/experimenter/experiments/api/v8/serializers.py
+++ b/experimenter/experimenter/experiments/api/v8/serializers.py
@@ -1,4 +1,5 @@
 import contextlib
+import datetime
 import json
 
 from django.conf import settings
@@ -106,10 +107,10 @@ class NimbusExperimentSerializer(serializers.ModelSerializer):
     outcomes = serializers.SerializerMethodField()
     segments = serializers.SerializerMethodField()
     startDate = serializers.DateField(source="start_date")
-    enrollmentEndDate = serializers.DateField(source="actual_enrollment_end_date")
-    endDate = serializers.DateField(source="end_date")
+    enrollmentEndDate = serializers.SerializerMethodField()
+    endDate = serializers.SerializerMethodField()
     proposedDuration = serializers.ReadOnlyField(source="proposed_duration")
-    proposedEnrollment = serializers.ReadOnlyField(source="proposed_enrollment")
+    proposedEnrollment = serializers.SerializerMethodField()
     referenceBranch = serializers.SerializerMethodField()
     featureValidationOptOut = serializers.ReadOnlyField(
         source="is_client_schema_disabled"
@@ -168,6 +169,26 @@ class Meta:
             "requiresRestart",
         )
 
+    def get_enrollmentEndDate(self, obj):
+        if obj.is_holdback and not obj._end_date and obj._enrollment_end_date:
+            return obj._enrollment_end_date
+        return obj.actual_enrollment_end_date
+
+    def get_endDate(self, obj):
+        if obj.is_holdback and not obj._end_date and obj._enrollment_end_date:
+            return obj._enrollment_end_date + datetime.timedelta(days=21)
+        return obj.end_date
+
+    def get_proposedEnrollment(self, obj):
+        if (
+            obj.is_holdback
+            and not obj.end_date
+            and obj._enrollment_end_date
+            and obj.start_date
+        ):
+            return (obj._enrollment_end_date - obj.start_date).days
+        return obj.proposed_enrollment
+
     def get_application(self, obj):
         return self.get_appId(obj)
 
diff --git a/experimenter/experimenter/experiments/models.py b/experimenter/experimenter/experiments/models.py
index 75af68d3f..547c7ef3f 100644
--- a/experimenter/experimenter/experiments/models.py
+++ b/experimenter/experimenter/experiments/models.py
@@ -3286,6 +3286,7 @@ class Messages:
         COMPLETED = "Experiment is complete"
         RESULTS_UPDATED = "Experiment results updated"
         MONITORING_DATA_UPDATED = "Experiment monitoring data updated"
+        HOLDBACK_ENROLLMENT_UPDATED = "Holdback enrollment period updated"
         EXPIRED_FROM_PREVIEW = "Expired from preview collection after 30 days"
         REMOVED_FROM_PREVIEW = "Removed from preview collection"
         PUSHED_TO_PREVIEW = "Pushed to preview collection"
diff --git a/experimenter/experimenter/jetstream/tasks.py b/experimenter/experimenter/jetstream/tasks.py
index 5078a4a7b..d13d4f9c9 100644
--- a/experimenter/experimenter/jetstream/tasks.py
+++ b/experimenter/experimenter/jetstream/tasks.py
@@ -198,6 +198,9 @@ def fetch_monitoring_data():
         raise
 
 
+HOLDBACK_OBSERVATION_DAYS = 21
+
+
 @app.task
 @metrics.timer_decorator("update_holdback_enrollment_period")
 def update_holdback_enrollment_period():
@@ -205,35 +208,34 @@ def update_holdback_enrollment_period():
     try:
         today = timezone.now().date()
         now = timezone.now()
+        enrollment_end = today - dt.timedelta(days=HOLDBACK_OBSERVATION_DAYS)
 
         experiments = NimbusExperiment.objects.filter(
             is_holdback=True,
             status=NimbusExperiment.Status.LIVE,
+            _end_date=None,
         ).exclude(_start_date=None)
 
         updated_count = 0
         for experiment in experiments:
-            enrollment_end = experiment.computed_enrollment_end_date
-            if enrollment_end is None or today <= enrollment_end:
+            if enrollment_end <= experiment.start_date:
                 continue
 
-            days_since_end = (today - enrollment_end).days
-            weeks_elapsed = max(1, days_since_end // 7)
-            new_enrollment_period = min(
-                experiment.proposed_enrollment + (weeks_elapsed * 7),
-                experiment.proposed_duration,
-            )
-
-            experiment.proposed_enrollment = new_enrollment_period
+            experiment._enrollment_end_date = enrollment_end
             experiment.do_rerun = True
             experiment.do_rerun_timestamp = now
             experiment.save(
                 update_fields=[
-                    "proposed_enrollment",
+                    "_enrollment_end_date",
                     "do_rerun",
                     "do_rerun_timestamp",
                 ]
             )
+            generate_nimbus_changelog(
+                experiment,
+                get_kinto_user(),
+                message=NimbusChangeLog.Messages.HOLDBACK_ENROLLMENT_UPDATED,
+            )
             updated_count += 1
 
         logger.info(
diff --git a/experimenter/experimenter/jetstream/tests/test_tasks.py b/experimenter/experimenter/jetstream/tests/test_tasks.py
index d92a08e5f..282d222cf 100644
--- a/experimenter/experimenter/jetstream/tests/test_tasks.py
+++ b/experimenter/experimenter/jetstream/tests/test_tasks.py
@@ -3952,23 +3952,9 @@ def test_result_is_cached(self):
 
 
 class TestUpdateHoldbackEnrollmentPeriod(TestCase):
-    def _make_holdback(self, start_date, enrollment_days, duration_days, end_date=None):
-        experiment = NimbusExperimentFactory.create_with_lifecycle(
-            NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
-            is_holdback=True,
-            _start_date=start_date,
-            proposed_enrollment=enrollment_days,
-            proposed_duration=duration_days,
-        )
-        if end_date:
-            experiment._computed_end_date = end_date
-            experiment.save(update_fields=["_computed_end_date"])
-        return experiment
-
-    def test_updates_enrollment_period_and_sets_do_rerun(self):
+    def test_sets_enrollment_end_date_and_do_rerun(self):
         today = datetime.date.today()
         start = today - datetime.timedelta(days=50)
-        # enrollment ended 36 days ago (14-day enrollment period started at start)
         experiment = NimbusExperimentFactory.create_with_lifecycle(
             NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
             is_holdback=True,
@@ -3979,55 +3965,56 @@ def test_updates_enrollment_period_and_sets_do_rerun(self):
         tasks.update_holdback_enrollment_period()
         experiment.refresh_from_db()
 
-        # weeks_elapsed = 36 // 7 = 5 → 14 + 35 = 49
-        self.assertEqual(experiment.proposed_enrollment, 49)
+        expected_enrollment_end = today - datetime.timedelta(days=21)
+        self.assertEqual(experiment._enrollment_end_date, expected_enrollment_end)
         self.assertTrue(experiment.do_rerun)
         self.assertIsNotNone(experiment.do_rerun_timestamp)
 
-    def test_skips_experiment_still_in_enrollment(self):
+    def test_skips_experiment_started_within_observation_period(self):
         today = datetime.date.today()
         experiment = NimbusExperimentFactory.create_with_lifecycle(
             NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
             is_holdback=True,
-            _start_date=today - datetime.timedelta(days=5),
+            _start_date=today - datetime.timedelta(days=10),
             proposed_enrollment=14,
             proposed_duration=84,
         )
-        original_enrollment = experiment.proposed_enrollment
         tasks.update_holdback_enrollment_period()
         experiment.refresh_from_db()
 
-        self.assertEqual(experiment.proposed_enrollment, original_enrollment)
+        self.assertIsNone(experiment._enrollment_end_date)
         self.assertFalse(experiment.do_rerun)
 
-    def test_skips_non_holdback_experiments(self):
+    def test_skips_ended_holdback(self):
         today = datetime.date.today()
         experiment = NimbusExperimentFactory.create_with_lifecycle(
             NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
-            is_holdback=False,
+            is_holdback=True,
             _start_date=today - datetime.timedelta(days=50),
             proposed_enrollment=14,
             proposed_duration=84,
         )
+        experiment._end_date = today - datetime.timedelta(days=1)
+        experiment.save(update_fields=["_end_date"])
         tasks.update_holdback_enrollment_period()
         experiment.refresh_from_db()
 
+        self.assertIsNone(experiment._enrollment_end_date)
         self.assertFalse(experiment.do_rerun)
 
-    def test_caps_enrollment_at_proposed_duration(self):
+    def test_skips_non_holdback_experiments(self):
         today = datetime.date.today()
         experiment = NimbusExperimentFactory.create_with_lifecycle(
             NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
-            is_holdback=True,
-            _start_date=today - datetime.timedelta(days=200),
+            is_holdback=False,
+            _start_date=today - datetime.timedelta(days=50),
             proposed_enrollment=14,
             proposed_duration=84,
         )
         tasks.update_holdback_enrollment_period()
         experiment.refresh_from_db()
 
-        self.assertEqual(experiment.proposed_enrollment, 84)
-        self.assertTrue(experiment.do_rerun)
+        self.assertFalse(experiment.do_rerun)
 
     def test_raises_on_unexpected_error(self):
         with (
diff --git a/experimenter/experimenter/settings.py b/experimenter/experimenter/settings.py
index 03d6b03f9..54d073eab 100644
--- a/experimenter/experimenter/settings.py
+++ b/experimenter/experimenter/settings.py
@@ -423,7 +423,7 @@
     },
     "update_holdback_enrollment_period": {
         "task": "experimenter.jetstream.tasks.update_holdback_enrollment_period",
-        "schedule": crontab(minute=0, hour=7, day_of_week=1),
+        "schedule": crontab(minute=0, hour=7),
     },
 }
 CELERY_TASK_ROUTES = {

From c83d3b8bbaf2cb9b6f8723145b74aafcd80c2589 Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Tue, 30 Jun 2026 16:00:58 -0700
Subject: [PATCH 07/10] chore(docs): update OpenAPI schema and Swagger UI

---
 docs/experimenter/openapi-schema.json | 6 ++----
 docs/experimenter/swagger-ui.html     | 6 ++----
 2 files changed, 4 insertions(+), 8 deletions(-)

diff --git a/docs/experimenter/openapi-schema.json b/docs/experimenter/openapi-schema.json
index 6e4ad500b..482f543cf 100644
--- a/docs/experimenter/openapi-schema.json
+++ b/docs/experimenter/openapi-schema.json
@@ -1647,11 +1647,11 @@
           },
           "enrollmentEndDate": {
             "type": "string",
-            "format": "date"
+            "readOnly": true
           },
           "endDate": {
             "type": "string",
-            "format": "date"
+            "readOnly": true
           },
           "proposedDuration": {
             "type": "string",
@@ -1711,8 +1711,6 @@
           "channel",
           "bucketConfig",
           "startDate",
-          "enrollmentEndDate",
-          "endDate",
           "publishedDate"
         ]
       },
diff --git a/docs/experimenter/swagger-ui.html b/docs/experimenter/swagger-ui.html
index 10cc5e59b..0fda98474 100644
--- a/docs/experimenter/swagger-ui.html
+++ b/docs/experimenter/swagger-ui.html
@@ -1659,11 +1659,11 @@
           },
           "enrollmentEndDate": {
             "type": "string",
-            "format": "date"
+            "readOnly": true
           },
           "endDate": {
             "type": "string",
-            "format": "date"
+            "readOnly": true
           },
           "proposedDuration": {
             "type": "string",
@@ -1723,8 +1723,6 @@
           "channel",
           "bucketConfig",
           "startDate",
-          "enrollmentEndDate",
-          "endDate",
           "publishedDate"
         ]
       },

From 5ae8bf21de39dca5409c30225c936695c881f9bf Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Tue, 30 Jun 2026 16:03:26 -0700
Subject: [PATCH 08/10] fix(nimbus): use queryset update to avoid protected
 attribute access on _enrollment_end_date

---
 experimenter/experimenter/jetstream/tasks.py | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/experimenter/experimenter/jetstream/tasks.py b/experimenter/experimenter/jetstream/tasks.py
index d13d4f9c9..31f83bc11 100644
--- a/experimenter/experimenter/jetstream/tasks.py
+++ b/experimenter/experimenter/jetstream/tasks.py
@@ -221,16 +221,12 @@ def update_holdback_enrollment_period():
             if enrollment_end <= experiment.start_date:
                 continue
 
-            experiment._enrollment_end_date = enrollment_end
-            experiment.do_rerun = True
-            experiment.do_rerun_timestamp = now
-            experiment.save(
-                update_fields=[
-                    "_enrollment_end_date",
-                    "do_rerun",
-                    "do_rerun_timestamp",
-                ]
+            NimbusExperiment.objects.filter(pk=experiment.pk).update(
+                _enrollment_end_date=enrollment_end,
+                do_rerun=True,
+                do_rerun_timestamp=now,
             )
+            experiment.refresh_from_db()
             generate_nimbus_changelog(
                 experiment,
                 get_kinto_user(),

From 5e9abd2256a44a7abf4524451b3610e065656cec Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Tue, 30 Jun 2026 16:07:32 -0700
Subject: [PATCH 09/10] fix(nimbus): use public properties and ISO format
 strings in holdback serializer methods

---
 .../experiments/api/v8/serializers.py         | 24 +++++++++----------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/experimenter/experimenter/experiments/api/v8/serializers.py b/experimenter/experimenter/experiments/api/v8/serializers.py
index ad8a63c5f..94af90da4 100644
--- a/experimenter/experimenter/experiments/api/v8/serializers.py
+++ b/experimenter/experimenter/experiments/api/v8/serializers.py
@@ -170,23 +170,21 @@ class Meta:
         )
 
     def get_enrollmentEndDate(self, obj):
-        if obj.is_holdback and not obj._end_date and obj._enrollment_end_date:
-            return obj._enrollment_end_date
-        return obj.actual_enrollment_end_date
+        enrollment_end = obj.actual_enrollment_end_date
+        if obj.is_holdback and not obj.end_date and enrollment_end:
+            return enrollment_end.isoformat()
+        return enrollment_end.isoformat() if enrollment_end else None
 
     def get_endDate(self, obj):
-        if obj.is_holdback and not obj._end_date and obj._enrollment_end_date:
-            return obj._enrollment_end_date + datetime.timedelta(days=21)
-        return obj.end_date
+        enrollment_end = obj.actual_enrollment_end_date
+        if obj.is_holdback and not obj.end_date and enrollment_end:
+            return (enrollment_end + datetime.timedelta(days=21)).isoformat()
+        return obj.end_date.isoformat() if obj.end_date else None
 
     def get_proposedEnrollment(self, obj):
-        if (
-            obj.is_holdback
-            and not obj.end_date
-            and obj._enrollment_end_date
-            and obj.start_date
-        ):
-            return (obj._enrollment_end_date - obj.start_date).days
+        enrollment_end = obj.actual_enrollment_end_date
+        if obj.is_holdback and not obj.end_date and enrollment_end and obj.start_date:
+            return (enrollment_end - obj.start_date).days
         return obj.proposed_enrollment
 
     def get_application(self, obj):

From 593685628c39cc8ae330e4aec922e2293efe3fe1 Mon Sep 17 00:00:00 2001
From: Yashika Khurana <yashikakhurana@Yashikas-MBP.home.local>
Date: Tue, 30 Jun 2026 16:12:09 -0700
Subject: [PATCH 10/10] test(nimbus): add coverage for holdback serializer
 method overrides

---
 .../tests/api/v8/test_serializers.py          | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/experimenter/experimenter/experiments/tests/api/v8/test_serializers.py b/experimenter/experimenter/experiments/tests/api/v8/test_serializers.py
index 1656fb868..8b76e09cb 100644
--- a/experimenter/experimenter/experiments/tests/api/v8/test_serializers.py
+++ b/experimenter/experimenter/experiments/tests/api/v8/test_serializers.py
@@ -419,6 +419,29 @@ def test_localized_localizations_json(self, l10n_json, expected):
         else:
             self.assertEqual(serializer.data["localizations"], expected)
 
+    def test_holdback_serializer_overrides(self):
+        today = datetime.date.today()
+        start = today - datetime.timedelta(days=50)
+        enrollment_end = today - datetime.timedelta(days=21)
+        experiment = NimbusExperimentFactory.create_with_lifecycle(
+            NimbusExperimentFactory.Lifecycles.LIVE_ENROLLING,
+            is_holdback=True,
+            _start_date=start,
+            _enrollment_end_date=enrollment_end,
+        )
+        serializer = NimbusExperimentSerializer(experiment)
+        data = serializer.data
+
+        self.assertEqual(data["enrollmentEndDate"], enrollment_end.isoformat())
+        self.assertEqual(
+            data["endDate"],
+            (enrollment_end + datetime.timedelta(days=21)).isoformat(),
+        )
+        self.assertEqual(
+            data["proposedEnrollment"],
+            (enrollment_end - start).days,
+        )
+
     def _experiment_data_without_branches_and_featureIds(
         self, experiment_data, min_required_version
     ) -> dict[str, Any]: