I am trying to subtract one date value from the value of datetime.datetime.today() to calculate how long ago something was. But it complains:

TypeError: can't subtract offset-naive and offset-aware date开发者_开发百科times

The return value from datetime.datetime.today() doesn't seem to be "timezone aware", while my other date value is. How do I get a return value from datetime.datetime.today() that is timezone aware?

The ideal solution would be for it to automatically know the timezone.

Right now, it's giving me the time in local time, which happens to be PST, i.e. UTC - 8 hours. Worst case, is there a way I can manually enter a timezone value into the datetime object returned by datetime.datetime.today() and set it to UTC-8?

In the standard library, there is no cross-platform way to create aware timezones without creating your own timezone class. (Edit: Python 3.9 introduces zoneinfo in the standard library which does provide this functionality.)

On Windows, there's win32timezone.utcnow(), but that's part of pywin32. I would rather suggest to use the pytz library, which has a constantly updated database of most timezones.

Working with local timezones can be very tricky (see "Further reading" links below), so you may rather want to use UTC throughout your application, especially for arithmetic operations like calculating the difference between two time points.

You can get the current date/time like so:

import pytz
from datetime import datetime
datetime.utcnow().replace(tzinfo=pytz.utc)

Mind that datetime.today() and datetime.now() return the local time, not the UTC time, so applying .replace(tzinfo=pytz.utc) to them would not be correct.

Another nice way to do it is:

datetime.now(pytz.utc)

which is a bit shorter and does the same.

Further reading/watching why to prefer UTC in many cases:

• pytz documentation
• What Every Developer Should Know About Time – development hints for many real-life use cases
• The Problem with Time & Timezones - Computerphile – funny, eye-opening explanation about the complexity of working with timezones (video)

Get the current time, in a specific timezone:

import datetime
import pytz
my_date = datetime.datetime.now(pytz.timezone('US/Pacific'))

Remember to install pytz first.

In Python 3.2+: datetime.timezone.utc:

The standard library makes it much easier to specify UTC as the time zone:

>>> import datetime
>>> datetime.datetime.now(datetime.timezone.utc)
datetime.datetime(2020, 11, 27, 14, 34, 34, 74823, tzinfo=datetime.timezone.utc)

You can also get a datetime that includes the local time offset using astimezone:

>>> datetime.datetime.now(datetime.timezone.utc).astimezone()
datetime.datetime(2020, 11, 27, 15, 34, 34, 74823, tzinfo=datetime.timezone(datetime.timedelta(seconds=3600), 'CET'))

(In Python 3.6+, you can shorten the last line to: datetime.datetime.now().astimezone())

If you want a solution that uses only the standard library and that works in both Python 2 and Python 3, see jfs' answer.

In Python 3.9+: zoneinfo to use the IANA time zone database:

In Python 3.9, you can specify particular time zones using the standard library, using zoneinfo, like this:

>>> from zoneinfo import ZoneInfo
>>> datetime.datetime.now(ZoneInfo("America/Los_Angeles"))
datetime.datetime(2020, 11, 27, 6, 34, 34, 74823, tzinfo=zoneinfo.ZoneInfo(key='America/Los_Angeles'))

zoneinfo gets its database of time zones from the operating system, or from the first-party PyPI package tzdata if available.

A one-liner using only the standard library works starting with Python 3.3. You can get a local timezone aware datetime object using astimezone (as suggested by johnchen902):

from datetime import datetime, timezone

aware_local_now = datetime.now(timezone.utc).astimezone()

print(aware_local_now)
# 2020-03-03 09:51:38.570162+01:00

print(repr(aware_local_now))
# datetime.datetime(2020, 3, 3, 9, 51, 38, 570162, tzinfo=datetime.timezone(datetime.timedelta(0, 3600), 'CET'))

Here's a stdlib solution that works on both Python 2 and 3:

from datetime import datetime

now = datetime.now(utc) # Timezone-aware datetime.utcnow()
today = datetime(now.year, now.month, now.day, tzinfo=utc) # Midnight

where today is an aware datetime instance representing the beginning of the day (midnight) in UTC and utc is a tzinfo object (example from the documentation):

from datetime import tzinfo, timedelta

ZERO = timedelta(0)

class UTC(tzinfo):
    def utcoffset(self, dt):
        return ZERO

    def tzname(self, dt):
        return "UTC"

    def dst(self, dt):
        return ZERO

utc = UTC()

Related: performance comparison of several ways to get midnight (start of a day) for a given UTC time. Note: it is more complex, to get midnight for a time zone with a non-fixed UTC offset.

Another method to construct time zone aware datetime object representing current time:

import datetime
import pytz

pytz.utc.localize( datetime.datetime.utcnow() )  

You can install pytz from PyPI by running:

$ pipenv install pytz

Use dateutil as described in Python datetime.datetime.now() that is timezone aware:

from dateutil.tz import tzlocal
# Get the current date/time with the timezone.
now = datetime.datetime.now(tzlocal())

If you are using Django, you can set dates non-tz aware (only UTC).

Comment the following line in settings.py:

USE_TZ = True

Here is one way to generate it with the stdlib:

import time
from datetime import datetime

FORMAT='%Y-%m-%dT%H:%M:%S%z'
date=datetime.strptime(time.strftime(FORMAT, time.localtime()),FORMAT)

date will store the local date and the offset from UTC, not the date at UTC timezone, so you can use this solution if you need to identify which timezone the date is generated at. In this example and in my local timezone:

date
datetime.datetime(2017, 8, 1, 12, 15, 44, tzinfo=datetime.timezone(datetime.timedelta(0, 7200)))

date.tzname()
'UTC+02:00'

The key is adding the %z directive to the representation FORMAT, to indicate the UTC offset of the generated time struct. Other representation formats can be consulted in the datetime module docs

If you need the date at the UTC timezone, you can replace time.localtime() with time.gmtime()

date=datetime.strptime(time.strftime(FORMAT, time.gmtime()),FORMAT)

date    
datetime.datetime(2017, 8, 1, 10, 23, 51, tzinfo=datetime.timezone.utc)

date.tzname()
'UTC'

Edit

This works only on python3. The z directive is not available on python 2 _strptime.py code

pytz is a Python library that allows accurate and cross platform timezone calculations using Python 2.3 or higher.

With the stdlib, this is not possible.

See a similar question on SO.

It should be emphasized that since Python 3.6, you only need the standard lib to get a timezone aware datetime object that represents local time (the setting of your OS). Using astimezone()

import datetime

datetime.datetime(2010, 12, 25, 10, 59).astimezone()
# e.g.
# datetime.datetime(2010, 12, 25, 10, 59, tzinfo=datetime.timezone(datetime.timedelta(seconds=3600), 'Mitteleuropäische Zeit'))

datetime.datetime(2010, 12, 25, 12, 59).astimezone().isoformat()
# e.g.
# '2010-12-25T12:59:00+01:00'

# I'm on CET/CEST

(see @johnchen902's comment).

Note there's a small caveat though, don't expect any "DST-awareness" from a timedelta timezone.

Here is a solution using a readable timezone and that works with today():

from pytz import timezone

datetime.now(timezone('Europe/Berlin'))
datetime.now(timezone('Europe/Berlin')).today()

You can list all timezones as follows:

import pytz

pytz.all_timezones
pytz.common_timezones # or

Getting a timezone-aware date in utc timezone is enough for date subtraction to work.

But if you want a timezone-aware date in your current time zone, tzlocal is the way to go:

from tzlocal import get_localzone  # pip install tzlocal
from datetime import datetime
datetime.now(get_localzone())

PS dateutil has a similar function (dateutil.tz.tzlocal). But inspite of sharing the name it has a completely different code base, which as noted by J.F. Sebastian can give wrong results.

Another alternative, in my mind a better one, is using Pendulum instead of pytz. Consider the following simple code:

>>> import pendulum

>>> dt = pendulum.now().to_iso8601_string()
>>> print (dt)
2018-03-27T13:59:49+03:00
>>>

To install Pendulum and see their documentation, go here. It have tons of options (like simple ISO8601, RFC3339 and many others format support), better performance and tend to yield simpler code.

Especially for non-UTC timezones:

The only timezone that has its own method is timezone.utc, but you can fudge a timezone with any UTC offset if you need to by using timedelta & timezone, and forcing it using .replace.

In [1]: from datetime import datetime, timezone, timedelta

In [2]: def force_timezone(dt, utc_offset=0):
   ...:     return dt.replace(tzinfo=timezone(timedelta(hours=utc_offset)))
   ...:

In [3]: dt = datetime(2011,8,15,8,15,12,0)

In [4]: str(dt)
Out[4]: '2011-08-15 08:15:12'

In [5]: str(force_timezone(dt, -8))
Out[5]: '2011-08-15 08:15:12-08:00'

Using timezone(timedelta(hours=n)) as the time zone is the real silver bullet here, and it has lots of other useful applications.

Tyler from 'howchoo' made a really great article that helped me get a better idea of the Datetime Objects, link below

Working with Datetime

essentially, I just added the following to the end of both my datetime objects

.replace(tzinfo=pytz.utc)

Example:

import pytz
import datetime from datetime

date = datetime.now().replace(tzinfo=pytz.utc)

If you get current time and date in python then import date and time,pytz package in python after you will get current date and time like as..

from datetime import datetime
import pytz
import time
str(datetime.strftime(datetime.now(pytz.utc),"%Y-%m-%d %H:%M:%S%t"))

Use the timezone as shown below for a timezone-aware date time. The default is UTC:

from django.utils import timezone
today = timezone.now()

try pnp_datetime, all the time been used and returned is with timezone, and will not cause any offset-naive and offset-aware issues.

>>> from pnp_datetime.pnp_datetime import Pnp_Datetime
>>>
>>> Pnp_Datetime.utcnow()
datetime.datetime(2020, 6, 5, 12, 26, 18, 958779, tzinfo=<UTC>)