python - Get timezone used by datetime.datetime.fromtimestamp()

Question

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.

Answer 1

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.