Celery Invalid format of 'date

The Fix

pip install celery==5.3.4

Based on closed celery/celery issue #8431 · PR/commit linked

Production note: This usually shows up under retries/timeouts. Treat it as a side-effect risk until you can verify behavior with a canary + real traffic.

Jump to Verify Open PR/Commit

@@ -182,7 +182,8 @@ def _store_result(self, task_id, result, state,
         """Store return value and state of an executed task."""
         meta = self._get_result_meta(result=self.encode(result), state=state,
-                                     traceback=traceback, request=request)
+                                     traceback=traceback, request=request,
+                                     format_date=False)

repro.py

software -> celery:5.3.1 (emerald-rush) kombu:5.3.1 py:3.8.16
            billiard:4.1.0 redis:4.6.0
platform -> system:Linux arch:64bit, ELF
            kernel version:5.19.0-46-generic imp:CPython
loader   -> celery.loaders.app.AppLoader
settings -> transport:redis results:mongodb
[...]
CELERY_BROKER_TRANSPORT_OPTIONS: 
 'socket_keepalive': True, 'socket_keepalive_options': {4: 600, 5: 60, 6: 5}}
CELERY_BROKER_URL: 'redis://redis.local:7000/0'
CELERY_INCLUDE: ['test_celery_result.tasks']
CELERY_QUEUE_NAME: 'test_celery_result'
CELERY_REDIS: 
 'host': 'redis.local', 'port': 7000}
[...]
is_overridden: <bound method Settings.is_overridden of <Settings "test_celery_result.settings">>
deprecated_settings: None
task_default_queue: 'test_celery_result'
enable_utc: False
result_backend: 'mongodb'
result_expires: datetime.timedelta(seconds=15)
mongodb_backend_settings: 
    'database': '********',
    'host': ['mongo-replica'],
    'port': 27017,
    'taskmeta_collection': 'celery_task_result'}
beat_schedule: 
    'celery.backend_cleanup': {   'schedule': 60,
                                  'task': 'celery.backend_cleanup'},
    'dummy_task': {'schedule': 15, 'task': 'dummy_task'}}

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install celery==5.3.4\nWhen NOT to use: Do not apply this fix if using a different result backend than MongoDB.\n\n

Why This Fix Works in Production

Trigger: - [x] I have included all related issues and possible duplicate issues
Mechanism: The 'date_done' field format in MongoDB backend was not being set correctly due to a missing argument
Why the fix works: Sets the `format_date` argument to `False` when calling `self.get_result_meta` in `MongoBackend._store_result` to fix the task_result collection not being cleaned up when the task_expires configuration is set. (first fixed release: 5.3.4).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

The 'date_done' field format in MongoDB backend was not being set correctly due to a missing argument
Production symptom (often without a traceback): - [x] I have included all related issues and possible duplicate issues

Proof / Evidence

GitHub issue: #8431
Fix PR: https://github.com/celery/celery/pull/8432
First fixed release: 5.3.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.46

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“ # Checklist  - [x] I hav”

Issue thread · issue description · source

Failure Signature (Search String)

- [x] I have included all related issues and possible duplicate issues
async-timeout==4.0.2

Copy-friendly signature

signature.txt

Failure Signature
-----------------
- [x] I have included all related issues and possible duplicate issues
async-timeout==4.0.2

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
- [x] I have included all related issues and possible duplicate issues
async-timeout==4.0.2

Minimal Reproduction

repro.py

software -> celery:5.3.1 (emerald-rush) kombu:5.3.1 py:3.8.16
            billiard:4.1.0 redis:4.6.0
platform -> system:Linux arch:64bit, ELF
            kernel version:5.19.0-46-generic imp:CPython
loader   -> celery.loaders.app.AppLoader
settings -> transport:redis results:mongodb
[...]
CELERY_BROKER_TRANSPORT_OPTIONS: 
 'socket_keepalive': True, 'socket_keepalive_options': {4: 600, 5: 60, 6: 5}}
CELERY_BROKER_URL: 'redis://redis.local:7000/0'
CELERY_INCLUDE: ['test_celery_result.tasks']
CELERY_QUEUE_NAME: 'test_celery_result'
CELERY_REDIS: 
 'host': 'redis.local', 'port': 7000}
[...]
is_overridden: <bound method Settings.is_overridden of <Settings "test_celery_result.settings">>
deprecated_settings: None
task_default_queue: 'test_celery_result'
enable_utc: False
result_backend: 'mongodb'
result_expires: datetime.timedelta(seconds=15)
mongodb_backend_settings: 
    'database': '********',
    'host': ['mongo-replica'],
    'port': 27017,
    'taskmeta_collection': 'celery_task_result'}
beat_schedule: 
    'celery.backend_cleanup': {   'schedule': 60,
                                  'task': 'celery.backend_cleanup'},
    'dummy_task': {'schedule': 15, 'task': 'dummy_task'}}

What Broke

Tasks were not being cleaned up from the result collection when task_expires was set.

Why It Broke

The 'date_done' field format in MongoDB backend was not being set correctly due to a missing argument

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install celery==5.3.4

When NOT to use: Do not apply this fix if using a different result backend than MongoDB.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Option D — Guard side-effects with OnceOnly Guardrail for side-effects

Mitigate duplicate external side-effects under retries/timeouts/agent loops by gating the operation before calling external systems.

Place OnceOnly between your code/agent and real side-effects (Stripe, emails, CRM, APIs).
Use a stable key per side-effect (e.g., customer_id + action + idempotency_key).
Fail-safe: configure fail-open vs fail-closed based on blast radius and spend risk.
This does NOT fix data corruption; it only prevents duplicate side-effects.

Show example snippet (optional)

onceonly.py

from onceonly import OnceOnly
import os

once = OnceOnly(api_key=os.environ["ONCEONLY_API_KEY"], fail_open=True)

# Stable idempotency key per real side-effect.
# Use a request id / job id / webhook delivery id / Stripe event id, etc.
event_id = "evt_..."  # replace
key = f"stripe:webhook:{event_id}"

res = once.check_lock(key=key, ttl=3600)
if res.duplicate:
    return {"status": "already_processed"}

# Safe to execute the side-effect exactly once.
handle_event(event_id)

See OnceOnly SDK

When NOT to use: Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Fix reference: https://github.com/celery/celery/pull/8432

First fixed release: 5.3.4

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not apply this fix if using a different result backend than MongoDB.
Do not use this to hide logic bugs or data corruption. Use it to block duplicate external side-effects and enforce tool permissions/spend caps.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Make timeouts explicit and test them (unit + integration) to avoid silent behavior changes.
Instrument retries (attempt count + reason) and alert on spikes to catch dependency slowdowns.

Version Compatibility Table

Version	Status
5.3.4	Fixed

Related Issues

No related fixes found.

Cluster: celery:data-consistency Celery hub Celery best practices All hubs All clusters

Related clusters: Configuration error Duplicates Duplicate processing

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

Celery Invalid format of 'date_done' field in celery.task_results with backend mongodb (Fix)