Pydantic datetime dumping to json flips sign of timezone

The Fix

pip install pydantic==1.10.1

Based on closed pydantic/pydantic issue #8964 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -1,5 +1,5 @@
 docutils==0.13.1
 Pygments==2.2.0
-Sphinx==1.6.2
+Sphinx==1.6.3
 sphinxcontrib-spelling==2.3.0

repro.py

from datetime import datetime

from dateutil.parser import isoparse
from pydantic import TypeAdapter

iso_str = "2024-03-06T14:21:34-00:08"
expected = isoparse(iso_str)

datetime_adapter = TypeAdapter(datetime)

actual_datetime = datetime_adapter.validate_python(iso_str)
assert actual_datetime == expected

python_dump = datetime_adapter.dump_python(actual_datetime, mode="python")
assert python_dump == expected, (python_dump, expected)

json_dump = datetime_adapter.dump_python(actual_datetime, mode="json")
assert json_dump == iso_str, (
    json_dump,
    iso_str,
)  # fails with ('2024-03-06T14:21:34+00:08', '2024-03-06T14:21:34-00:08')
#                                    ^                            ^

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 pydantic==1.10.1\nWhen NOT to use: Do not use this fix if the application relies on the incorrect timezone behavior.\n\n

Why This Fix Works in Production

Trigger: assert actual_datetime == expected
Mechanism: The datetime dumping to JSON incorrectly flips the sign of the timezone
Why the fix works: Fixed a bug where datetime dumping to JSON flipped the sign of the timezone. (first fixed release: 1.10.1).

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 datetime dumping to JSON incorrectly flips the sign of the timezone
Production symptom (often without a traceback): assert actual_datetime == expected

Proof / Evidence

GitHub issue: #8964
Fix PR: https://github.com/pydantic/speedate/pull/56
First fixed release: 1.10.1
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.70
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.51

Discussion

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

Jump to Sources Open on GitHub

“@FynnBe, Thanks for reporting! Indeed, this is a bug. Probably needs to be fixed in pydantic-core. I'll take a look today!”

@sydney-runkle · 2024-03-07 · confirmation · source

“Fixed this in speedate, will be released with pydantic v2.7.0!”

@sydney-runkle · 2024-03-12 · confirmation · source

“I can confirm that this is a bug in how speedate converts timezone info objects to strings :). I'll contribute a fix soon!”

@sydney-runkle · 2024-03-07 · source

Failure Signature (Search String)

assert actual_datetime == expected
assert python_dump == expected, (python_dump, expected)

Copy-friendly signature

signature.txt

Failure Signature
-----------------
assert actual_datetime == expected
assert python_dump == expected, (python_dump, expected)

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
assert actual_datetime == expected
assert python_dump == expected, (python_dump, expected)

Minimal Reproduction

repro.py

from datetime import datetime

from dateutil.parser import isoparse
from pydantic import TypeAdapter

iso_str = "2024-03-06T14:21:34-00:08"
expected = isoparse(iso_str)

datetime_adapter = TypeAdapter(datetime)

actual_datetime = datetime_adapter.validate_python(iso_str)
assert actual_datetime == expected

python_dump = datetime_adapter.dump_python(actual_datetime, mode="python")
assert python_dump == expected, (python_dump, expected)

json_dump = datetime_adapter.dump_python(actual_datetime, mode="json")
assert json_dump == iso_str, (
    json_dump,
    iso_str,
)  # fails with ('2024-03-06T14:21:34+00:08', '2024-03-06T14:21:34-00:08')
#                                    ^                            ^

Environment

Pydantic: 2

What Broke

JSON output shows incorrect timezone sign, leading to potential data misinterpretation.

Why It Broke

The datetime dumping to JSON incorrectly flips the sign of the timezone

Fix Options (Details)

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

pip install pydantic==1.10.1

When NOT to use: Do not use this fix if the application relies on the incorrect timezone behavior.

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

Fix reference: https://github.com/pydantic/speedate/pull/56

First fixed release: 1.10.1

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

When NOT to Use This Fix

Do not use this fix if the application relies on the incorrect timezone behavior.

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

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.

Version Compatibility Table

Version	Status
1.10.1	Fixed

Related Issues

No related fixes found.

Cluster: pydantic:data-consistency Pydantic hub Pydantic best practices All hubs All clusters

Related clusters: Configuration error OpenAPI schema Serialization

Sources

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

Pydantic datetime dumping to json flips sign of timezone (Fix)