Is it possible, and if yes, how, to get the time zone (i.e. the UTC offset or a datetime.timezone instance with that offset) that is used by datetime.datetime.fromtimestamp() to convert a POSIX timestamp (seconds since the epoch) to a datetime object?

datetime.datetime.fromtimestamp() converts a POSIX timestamp to a naive datetime object (i.e. without a tzinfo), but does so using the system’s locale to adjust it to the local timezone and the UTC offset that was in effect at that time.

For example, using the date 2008-12-27 midnight UTC (40 * 356 * 86400 seconds since the epoch):

>>> datetime.datetime.fromtimestamp(40 * 356 * 86400)
datetime.datetime(2008, 12, 27, 1, 0)

That timestamp is converted to a datetime object at 1 o’clock in the morning (which it was at that time, here in an CET/CEST timezone). 100 days later, this is the result:

>>> datetime.datetime.fromtimestamp((40 * 356 + 100) * 86400)
datetime.datetime(2009, 4, 6, 2, 0)

Which is 2 o’clock in the morning. This is because by then, DST was active.

I’d expected that datetime.datetime.fromtimestamp() would set the tzinfo it uses in the returned datetime instance, but it doesn’t.

datetime.fromtimestamp(ts) converts “seconds since the epoch” to a naive datetime object that represents local time. tzinfo is always None in this case.

Local timezone may have had a different UTC offset in the past. On some systems that provide access to a historical timezone database, fromtimestamp() may take it into account.

To get the UTC offset used by fromtimestamp():

utc_offset = fromtimestamp(ts) - utcfromtimestamp(ts)

See also, Getting computer’s utc offset in Python.

From the Python documentation:

classmethod datetime.fromtimestamp(timestamp, tz=None)

Return the local date and time corresponding to the POSIX timestamp, such as is returned by time.time(). If optional argument tz is None or not specified, the timestamp is converted to the platform’s local date and time, and the returned datetime object is naive.

Else tz must be an instance of a class tzinfo subclass, and the timestamp is converted to tz‘s time zone. In this case the result is equivalent to tz.fromutc(datetime.utcfromtimestamp(timestamp).replace(tzinfo=tz)).

The key part of this description as it relates to your question is that when you don’t specify a time zone, not only does it use the local time zone, but the result is naive. You seem to want it to be aware.

This is a particular distinction made by Python, and is discussed right at the very top of the datetime documentation.

If what you want is a datetime that is aware of the local time zone, try the tzlocal library. It is focused on that particular problem. See also this question.

If you know the timezone of the timestamp you want to convert, you can simply send it in while calling fromtimestamp:

>>> from datetime import datetime
>>> import pytz
>>> datetime.fromtimestamp(1562684265, pytz.timezone("Europe/Stockholm"))
datetime.datetime(2019, 7, 9, 16, 57, 45, tzinfo=<DstTzInfo 'Europe/Stockholm' CEST+2:00:00 DST>)
>>> datetime.fromtimestamp(1562684265, pytz.timezone("UTC"))
datetime.datetime(2019, 7, 9, 14, 57, 45, tzinfo=<UTC>)

Using time.gmtime you can extract the timezone as described in this previous answer: Get TZ information of the system in Python?.

>>> from __future__ import print_function
>>> from time import gmtime, strftime
>>> print(strftime("%z", gmtime()))

Prints -06:00 for my CST laptop in both python-2.7 and python-3.3
You can also use localtime() to get a local time struct.

>>> from __future__ import print_function
>>> from time import localtime
>>> lt = localtime()
>>> print(lt.tm_zone)
>>> print(lt.tm_gmtoff/(60*60))
>>> print(lt.tm_gmtoff/(60*60) - (1 if lt.tm_isdst == 1 else 0)) # Adjusted for DST

Hope this helps