Converting timezone-aware datetime to local time in Python

Question

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?

Boa

related: How to convert a python utc datetime to a local datetime using only python standard library? – Worl

Answer 1

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)

Tenorrhaphy

Yes, I did ultimately decide to store the dates as naive UTC. You've answered both questions I had about that (stripping tzinfo and converting timezone), so thanks! – Boa

fromtimestamp() may fail for dates from the past (if UTC offset was different at the time in the local timezone) if it doesn't use the tz database to find UTC offset (It does on Ubuntu, I'm not sure about Windows). – Worl

Answer 2

In recent versions of Django (at least 1.4.1):

from django.utils.timezone import localtime

result = localtime(some_time_object)

Whereby

localtime() may use settings.TIME_ZONE that may be different from the local timezone e.g., if you haven't set it then it is 'America/Chicago' by default. If localtime() does not use settings.TIME_ZONE then it uses broken (according to its documentation) LocalTimezone() – Worl

Answer 3

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.

Worl

Answer 4

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.

Susan

dateutil-based code may fail for ambiguous local times – Worl

See that the whole issue of PEP-0495 Local Time Dissambiguation is a bit hairy in general. After reading issue #57 I understand that tzlocal is smarter and tries to figure out your real system timezone settings to dissambiguate, which probably is what most of the people would like. – Susan

It is not about how smart they are: both tzlocal.get_localzone() and dateutil.tz.tzlocal() may load their data from the same tzdata file. The bugs are due to an inherit limitation in dateutil approach to how timezone-aware datetime objects are interpreted (this approach requires PEP 495 to disambiguate even unambiguous conversions such as utc -> local). pytz works today (get_localzone() returns pytz tzinfo objects). – Worl

ValueError: astimezone() cannot be applied to a naive datetime – Roswell

Answer 5

I use this helper function all the time.

from datetime import datetime
import pytz

def tz_convert(t: datetime, tz=pytz.utc):
    '''
    Convert a timestamp to the target timezone.

    If the timestamp is naive, the timezone is set to the target timezone.
    '''
    if not t.tzinfo:
        tc = t.replace(tzinfo=tz)
    else:
        tc = t.astimezone(tz)

    return tc

Demo

# tz-aware timestamp
>>> t = datetime.now(tz=pytz.utc)
>>> t.isoformat()
'2022-09-15T08:24:38.093312+00:00'
>>> tc = tz_convert(t, pytz.timezone('est'))
>>> tc.isoformat()
'2022-09-15T03:24:38.093312-05:00'

# tz-naive timestamp
>>> t = datetime.now()
>>> t.isoformat()
'2022-09-15T10:22:41.464200'
>>> tc = tz_convert(t, pytz.timezone('est'))
>>> tc.isoformat()
'2022-09-15T10:22:41.464200-05:00'

Busiek

Note that you can pass None as the tz argument to convert tz-aware timestamps to the local timezone. – Busiek