Converting timezone-aware datetime to local time in Python

How do you convert a timezone-aware datetime object to the equivalent non-timezone-aware datetime for the local timezone?

My particular application uses Django (although, this is in reality a generic Python question):

import iso8601

....

date_str="2010-10-30T17:21:12Z"

....

d = iso8601.parse_date(date_str)

foo = app.models.FooModel(the_date=d)
foo.save()

This causes Django to throw an error:

raise ValueError("MySQL backend does not support timezone-aware datetimes.")

What I need is:

d = iso8601.parse_date(date_str)
local_d = SOME_FUNCTION(d)
foo = app.models.FooModel(the_date=local_d)

What would SOME_FUNCTION be?

In general, to convert an arbitrary timezone-aware datetime to a naive (local) datetime, I'd use the pytz module and astimezone to convert to local time, and replace to make the datetime naive:

In [76]: import pytz

In [77]: est=pytz.timezone('US/Eastern')

In [78]: d.astimezone(est)
Out[78]: datetime.datetime(2010, 10, 30, 13, 21, 12, tzinfo=<DstTzInfo 'US/Eastern' EDT-1 day, 20:00:00 DST>)

In [79]: d.astimezone(est).replace(tzinfo=None)
Out[79]: datetime.datetime(2010, 10, 30, 13, 21, 12)

But since your particular datetime seems to be in the UTC timezone, you could do this instead:

In [65]: d
Out[65]: datetime.datetime(2010, 10, 30, 17, 21, 12, tzinfo=tzutc())

In [66]: import datetime

In [67]: import calendar

In [68]: datetime.datetime.fromtimestamp(calendar.timegm(d.timetuple()))
Out[68]: datetime.datetime(2010, 10, 30, 13, 21, 12)

By the way, you might be better off storing the datetimes as naive UTC datetimes instead of naive local datetimes. That way, your data is local-time agnostic, and you only convert to local-time or any other timezone when necessary. Sort of analogous to working in unicode as much as possible, and encoding only when necessary.

So if you agree that storing the datetimes in naive UTC is the best way, then all you'd need to do is define:

local_d = d.replace(tzinfo=None)

In recent versions of Django (at least 1.4.1):

from django.utils.timezone import localtime

result = localtime(some_time_object)

A portable robust solution should use the tz database. To get local timezone as pytz tzinfo object, use tzlocal module:

#!/usr/bin/env python
import iso8601
import tzlocal # $ pip install tzlocal

local_timezone = tzlocal.get_localzone()
aware_dt = iso8601.parse_date("2010-10-30T17:21:12Z") # some aware datetime object
naive_local_dt = aware_dt.astimezone(local_timezone).replace(tzinfo=None)

Note: it might be tempting to use something like:

#!/usr/bin/env python3
# ...
naive_local_dt = aware_dt.astimezone().replace(tzinfo=None)

but it may fail if the local timezone has a variable utc offset but python does not use a historical timezone database on a given platform.

Using python-dateutil you can parse the date in iso-8561 format with dateutil.parsrser.parse() that will give you an aware datetime in UTC/Zulu timezone.

Using .astimezone() you can convert it to an aware datetime in another timezone.

Using .replace(tzinfo=None) will convert the aware datetime into a naive datetime.

from datetime import datetime
from dateutil import parser as datetime_parser
from dateutil.tz import tzutc,gettz

aware = datetime_parser.parse('2015-05-20T19:51:35.998931Z').astimezone(gettz("CET"))
naive = aware.replace(tzinfo=None)

In general the best idea is to convert all dates to UTC and store them that way, and convert them back to local as needed. I use aware.astimezone(tzutc()).replace(tzinfo=None) to make sure is in UTC and convert to naive.