CODEWITHSUNDEEP
pythondatetimetimezonepytz
jxstanford
jxstanford

Reputation: 3387

unexpected results converting timezones in python

I'm trying to understand why I'm getting these results when converting time zones to UTC:

In [74]: d1 = datetime(2007, 12, 5, 6, 30,tzinfo=pytz.timezone('US/Pacific'))
In [75]: d1
Out[75]: datetime.datetime(2007, 12, 5, 6, 30, tzinfo=<DstTzInfo 'US/Pacific' LMT-1 day, **16:07:00 STD**>)
In [76]: d1.astimezone(pytz.utc)
Out[76]: datetime.datetime(2007, 12, 5, 14, 23, tzinfo=<UTC>)

Why did 6:30am become 2:23pm?

On the other hand, if I use the following approach, I get the expected result:

In [90]: d2 = datetime(2007, 12, 5, 6, 30)
In [91]: uspac = pytz.timezone('US/Pacific')
In [92]: d2_aware = uspac.localize(d2)
In [94]: d2_aware.astimezone(pytz.utc)
Out[94]: datetime.datetime(2007, 12, 5, 14, 30, tzinfo=<UTC>)

Upvotes: 59

Views: 16119

Answers (6)

Martim
Martim

Reputation: 1086

As of Python 3.9, the recommended way is to create datetime with timezone info by using datetime(), contrary to what was the case when the currently top-rated answer was posted. However, instead of using the 3rd party library pytz, you should now instead use zoneinfo from the standard library.

>>> from zoneinfo import ZoneInfo
>>> from datetime import datetime, timedelta

>>> dt = datetime(2020, 10, 31, 12, tzinfo=ZoneInfo("America/Los_Angeles"))
>>> print(dt)
2020-10-31 12:00:00-07:00

>>> dt.tzname()
'PDT'

Upvotes: 0

Shadow
Shadow

Reputation: 9427

Unfortunately, creating timezone aware dates using this method doesn't work.

If you are using Django, they have a utility function, make_aware, that does this correctly.

from django.utils.timezone import make_aware
from pytz import timezone

unaware_datetime = datetime(2007, 12, 5)
local_datetime = make_aware(datetime(2007, 12, 5))
specific_datetime = make_aware(datetime(2007, 12, 5), timezone("Australia/Melbourne"))

If you're not using Django, then the source code for the make_aware function may give you inspiration.

Upvotes: 9

Bill Bell
Bill Bell

Reputation: 21643

I'm revisiting some questions about date and time to see if some of the newer libraries prove more helpful in situations like this (or not). pendulum is one that stores timezone with date and time, making it particularly valuable in situations such as this.

>>> import pendulum
>>> d1 = pendulum.datetime(2007,12,5,6,30, tzinfo='US/Pacific')
>>> d1
<Pendulum [2007-12-05T06:30:00-08:00]>
>>> d1.timezone
<Timezone [US/Pacific]>
>>> d1.astimezone(tz='UTC')
<Pendulum [2007-12-05T14:30:00+00:00]>

Lots of other sweet features too.

Upvotes: 4

Kenial
Kenial

Reputation: 2540

What I got is just a workaround, the simple rule is Never create datetime with timezone info by using datetime().

This sample would give you a hint for this. As you see, you could avoid the unexpected difference, once and only you make "naive" datetime (it is, datetime without timezone info) and then localize it (it is not applied when you create datetime on UTC though) :

import pytz
from datetime import datetime

# make Jan 1 on PDT -> UTC
pdt = pytz.timezone("America/Los_Angeles")
pdtnow1 = datetime(2014,1,1, tzinfo=pdt)
pdtnow2 = pdt.localize(datetime(2014,1,1))
pytz.utc.normalize(pdtnow1)
# > datetime.datetime(2014, 1, 1, 7, 53, tzinfo=<UTC>)
pytz.utc.normalize(pdtnow2)
# > datetime.datetime(2014, 1, 1, 8, 0, tzinfo=<UTC>)

# make Jan 1 on UTC -> PDT
utcnow1 = datetime(2014,1,1, tzinfo=pytz.utc)
utcnow2 = pytz.utc.localize(datetime(2014,1,1))
pdt.normalize(utcnow1)
# > datetime.datetime(2013, 12, 31, 16, 0, 
# > tzinfo=<DstTzInfo 'America/Los_Angeles' PST-1 day, 16:00:00 STD>)
pdt.normalize(utcnow2)
# > datetime.datetime(2013, 12, 31, 16, 0, 
# > tzinfo=<DstTzInfo 'America/Los_Angeles' PST-1 day, 16:00:00 STD>)

Upvotes: 66

user3834473
user3834473

Reputation:

From the partial documentation: http://pytz.sourceforge.net/#localized-times-and-date-arithmetic

Unfortunately using the tzinfo argument of the standard datetime constructors ‘’does not work’’ with pytz for many timezones. [...] It is safe for timezones without daylight saving transitions though, such as UTC. [...] The preferred way of dealing with times is to always work in UTC, converting to localtime only when generating output to be read by humans.

Upvotes: 43

furas
furas

Reputation: 142641

Print d2_aware before .astimezone and you see PST-1 (Pacific Standard Time) but in first example you have LMT-1 (Local Mean Time) - and probably it can give 7 minutes difference.

But I don't know why pytz use different timezones.

Upvotes: 0

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