Celery crontab() schedule executed one hour too late after

The Fix

pip install celery==5.5.2

Based on closed celery/celery issue #9653 · 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

@@ -204,7 +204,7 @@ def delta_resolution(dt: datetime, delta: timedelta) -> datetime:
         start: datetime, ends_in: timedelta, now: Callable | None = None,
         relative: bool = False) -> timedelta:
-    """Calculate the remaining time for a start date and a timedelta.
+    """Calculate the real remaining time for a start date and a timedelta.

repro.py

import pytz
from datetime import datetime
from datetime import timedelta

from celery.schedules import crontab

timezone = pytz.timezone("Europe/Zurich")

def ts(s: str) -> datetime:
    return timezone.localize(datetime.fromisoformat(s))

now = ts("2025-03-30 00:00:02")
c = crontab(hour=0, minute=0, nowfun=lambda: now)

res = c.is_due(ts("2025-03-30 00:00:01"))
print(res)
# schedstate(is_due=False, next=86398.0)
print((now + timedelta(seconds=res.next)).astimezone(timezone).isoformat())
# 2025-03-31T01:00:00+02:00

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==5.5.2\nWhen NOT to use: This fix is not applicable if the application does not rely on accurate scheduling across DST changes.\n\n

Why This Fix Works in Production

Trigger: - [x] I have included all related issues and possible duplicate issues
Mechanism: The calculation of remaining time did not account for Daylight Saving Time changes
Why the fix works: Fix calculating the remaining time across DST changes. (first fixed release: 5.5.2).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

The calculation of remaining time did not account for Daylight Saving Time changes
Production symptom (often without a traceback): - [x] I have included all related issues and possible duplicate issues

Proof / Evidence

GitHub issue: #9653
Fix PR: https://github.com/celery/celery/pull/9669
First fixed release: 5.5.2
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.49

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
or possible duplicates to this issue as requested by the checklist above.

Copy-friendly signature

signature.txt

Failure Signature
-----------------
- [x] I have included all related issues and possible duplicate issues
or possible duplicates to this issue as requested by the checklist above.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
- [x] I have included all related issues and possible duplicate issues
or possible duplicates to this issue as requested by the checklist above.

Minimal Reproduction

repro.py

import pytz
from datetime import datetime
from datetime import timedelta

from celery.schedules import crontab

timezone = pytz.timezone("Europe/Zurich")

def ts(s: str) -> datetime:
    return timezone.localize(datetime.fromisoformat(s))

now = ts("2025-03-30 00:00:02")
c = crontab(hour=0, minute=0, nowfun=lambda: now)

res = c.is_due(ts("2025-03-30 00:00:01"))
print(res)
# schedstate(is_due=False, next=86398.0)
print((now + timedelta(seconds=res.next)).astimezone(timezone).isoformat())
# 2025-03-31T01:00:00+02:00

What Broke

Scheduled tasks executed one hour later than expected after DST changes.

Why It Broke

The calculation of remaining time did not account for Daylight Saving Time changes

Fix Options (Details)

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

pip install celery==5.5.2

When NOT to use: This fix is not applicable if the application does not rely on accurate scheduling across DST changes.

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

Fix reference: https://github.com/celery/celery/pull/9669

First fixed release: 5.5.2

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

When NOT to Use This Fix

This fix is not applicable if the application does not rely on accurate scheduling across DST changes.

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

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
5.5.2	Fixed

Related Issues

No related fixes found.

Cluster: celery:configuration-error Celery hub Celery best practices All hubs All clusters

Related clusters: Data consistency Duplicates Duplicate processing

Sources

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

Celery crontab() schedule executed one hour too late after DST start (Fix)