unexpected behavior of `datetime.astimezone` method

[donBarbos]

Bug report

Bug description:

my system time zone is UTC+04:00

>>> import datetime
>>> datetime.datetime(9999, 12, 31).astimezone() # same for .astimezone(datetime.UTC)
Traceback (most recent call last):
  File "<python-input-46>", line 1, in <module>
    datetime.datetime(9999, 12, 31).astimezone()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
ValueError: year must be in 1..9999, not 10000
>>> datetime.datetime(9999, 12, 31, tzinfo=datetime.UTC).astimezone()
datetime.datetime(9999, 12, 31, 4, 0, tzinfo=datetime.timezone(datetime.timedelta(seconds=14400), '+04'))

same behavior for _pydatetime module, but I found that in version 3.11 the behavior is as next:

>>> datetime.datetime(9999, 12, 31).astimezone()
datetime.datetime(9999, 12, 31, 0, 0, tzinfo=datetime.timezone(datetime.timedelta(seconds=14400), '+04'))

EDIT: same behavior for datetime(1, 1, 1):

>>> datetime.datetime(1, 1, 1).astimezone()
Traceback (most recent call last):
  File "<python-input-23>", line 1, in <module>
    datetime.datetime(1, 1, 1).astimezone()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
ValueError: year must be in 1..9999, not 0

CPython versions tested on:

CPython main branch, 3.13, 3.12

Operating systems tested on:

Linux

Linked PRs

• gh-130718: Normalize edge cases in datetime.timestamp and datetime.astimezone #130752

[picnixz]

We might have some broader checks or maybe there's some rollover somewhere, or we somehow missed a bound check

[donBarbos]

Where does the problem arise?

1. local_to_seconds function in datetimemodule.c for c impl
2. _mktime method in _pydatetime.py for python impl

they have same logic and use local() function inside. in both implementations it is called the same

Where the problematic code is used?

We have two methods that raising this error: datetime.astimezone and datetime.timestamp

(error in timestamp method is reproduced exactly the same as in my first example but you should to add a certain number of hours. sorry, I forgot how many exactly)