datetime.datetime.now() returns wrong time on Windows when env var TZ is set

[jjYBdx4IL]

Bug report

Bug description:

Europe/Berlin is UTC+2 right now, not UTC+1

Works fine on Linux OSes (builds).

C:\Users\work>python
Python 3.12.6 (tags/v3.12.6:a4a2d2b, Sep  6 2024, 20:11:23) [MSC v.1940 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> os.environ['TZ'] = 'Europe/Berlin'
>>> import datetime
>>> datetime.datetime.now()
datetime.datetime(2024, 9, 20, 12, 15, 24, 771320)
>>> datetime.datetime.now(tz=datetime.UTC)
datetime.datetime(2024, 9, 20, 11, 15, 45, 162005, tzinfo=datetime.timezone.utc)
>>>

CPython versions tested on:

3.12

Operating systems tested on:

Windows

[donBarbos]

Time zones really work strangely.
It's february now and in Berlin they use UTC+1 but i got next:

>>> os.environ['TZ'] = 'Europe/Berlin'
>>> datetime.datetime.now() # my timezone UTC+4 and this is correct
datetime.datetime(2025, 2, 26, 2, 48, 1, 560149)
>>> datetime.datetime.now(tz=datetime.UTC) # current tz in Berlin: CET — Central European Time
datetime.datetime(2025, 2, 25, 22, 47, 59, 926276, tzinfo=datetime.timezone.utc) # should be 23 hourse

tested on Linux and using main branch

[ttw225]

TL;DR

This issue is most likely caused by an outdated local timezone database, rather than a bug in CPython itself.

✅ Python officially recommends using the tzdata package as defined in PEP 615
to ensure accurate and up-to-date timezone data.

---

Investigating Timezone Data Source in zoneinfo

ZoneInfo Data Source Logic

The internal loading mechanism in CPython is as follows:
https://github.com/python/cpython/blob/main/Lib/zoneinfo/_zoneinfo.py#L61-L75

@classmethod
def _new_instance(cls, key):
    obj = super().__new__(cls)
    obj._key = key
    obj._file_path = obj._find_tzfile(key)

    if obj._file_path is not None:
        file_obj = open(obj._file_path, "rb")
    else:
        file_obj = _common.load_tzdata(key)

    with file_obj as f:
        obj._load_file(f)

    return obj

Priority of data source:

1. Check system timezone database (TZPATH)
2. Fallback to tzdata package if not found

Basic Test: Check Source and Offset

import os
from datetime import datetime
from zoneinfo import ZoneInfo
from zoneinfo._tzpath import TZPATH
from typing import Optional

def get_local_tz_path(timezone: str) -> Optional[str]:
    for path in TZPATH:
        full_path = os.path.join(path, timezone)
        if os.path.exists(full_path):
            return full_path
    return None

def show_basic_timezone_info(timezone: str) -> None:
    try:
        tz = ZoneInfo(timezone)
        now = datetime.now(tz)
        offset = now.utcoffset()
        offset_hours = offset.total_seconds() / 3600 if offset else 0.0
        local_path = get_local_tz_path(timezone)
        source_info = local_path if local_path else "Not from local TZ database"

        print(f"Timezone: {timezone}")
        print(f"Current time: {now}")
        print(f"UTC offset: UTC{offset_hours:+.1f}")
        print(f"Source: {source_info}")
    except Exception as e:
        print(f"Error: {e}")

Example usage

>>> show_basic_timezone_info("Europe/Berlin")
Timezone: Europe/Berlin
Current time: 2025-06-15 05:30:19.564745+02:00
UTC offset: UTC+2.0
Source: /usr/share/zoneinfo/Europe/Berlin

---

Force tzdata for Testing

Use the following to force tzdata usage instead of local TZPATH:

import os
os.environ["PYTHONTZPATH"] = ""

from zoneinfo import ZoneInfo
from datetime import datetime

timezone = "Europe/Berlin"
tz = ZoneInfo(timezone)
now = datetime.now(tz)
offset = now.utcoffset()
offset_hours = offset.total_seconds() / 3600 if offset else 0.0

from zoneinfo._tzpath import TZPATH
def detect_timezone_source(name: str) -> str:
    for path in TZPATH:
        if os.path.exists(os.path.join(path, name)):
            return f"System TZ database: {os.path.join(path, name)}"
    try:
        import tzdata
        tzdata_path = os.path.join(os.path.dirname(tzdata.__file__), "zoneinfo", name)
        if os.path.exists(tzdata_path):
            return f"tzdata package: {tzdata_path}"
    except ImportError:
        pass
    return "Unknown source"

print(f"Timezone: {timezone}")
print(f"Current time: {now.isoformat()}")
print(f"UTC offset: UTC{offset_hours:+.1f}")
print(f"Source: {detect_timezone_source(timezone)}")

Output:

Timezone: Europe/Berlin
Current time: 2025-06-15T10:25:25.895861+02:00
UTC offset: UTC+2.0
Source: tzdata package: /Users/user/.venv/lib/python3.13/site-packages/tzdata/zoneinfo/Europe/Berlin

Summary

• ZoneInfo prioritizes system tzdata over tzdata package
• Ensuring your system tzdata is up to date, or explicitly using tzdata, helps avoid inconsistencies