Bug: Cookie Expires labels a non-UTC aware datetime as GMT without converting it #8671

Expected Behaviour

A Cookie whose expires is a timezone-aware datetime should send that instant in GMT, as RFC 6265 requires for Expires. datetime(2026, 1, 1, 12, tzinfo=timezone(timedelta(hours=2))) is 10:00 UTC, so the header should read Expires=Thu, 01 Jan 2026 10:00:00 GMT.

Current Behaviour

Cookie._format_date formats the datetime with strftime and a hard-coded GMT suffix, without converting it to UTC first. The local wall-clock time is labelled GMT, so the cookie expires earlier or later by the UTC offset: Expires=Thu, 01 Jan 2026 12:00:00 GMT.

Code snippet

from datetime import datetime, timedelta, timezone
from aws_lambda_powertools.shared.cookies import Cookie

cookie = Cookie("a", "b", expires=datetime(2026, 1, 1, 12, tzinfo=timezone(timedelta(hours=2))))
print(str(cookie))  # a=b; Secure; Expires=Thu, 01 Jan 2026 12:00:00 GMT

Possible Solution

Convert an aware datetime to UTC before formatting (expires.astimezone(timezone.utc)); naive datetimes keep today's behaviour. I have this fix with a regression test in tests/unit/test_cookie_class.py (fails on develop, passes with the change) and can open a PR once this is triaged.

Steps to Reproduce

Run the snippet above on develop (5785377).

Powertools for AWS Lambda (Python) version

latest (3.37.0, develop)

AWS Lambda function runtime

3.13

Packaging format used

PyPi

Debugging logs

N/A