pytz 2009u

Introduction

pytz brings the Olson tz database into Python. This library allows accurate and cross platform timezone calculations using Python 2.3 or higher. It also solves the issue of ambiguous times at the end of daylight savings, which you can read more about in the Python Library Reference (datetime.tzinfo).

Amost all of the Olson timezones are supported.

Note that this library differs from the documented Python API for tzinfo implementations; if you want to create local wallclock times you need to use the localize() method documented in this document. In addition, if you perform date arithmetic on local times that cross DST boundaries, the results may be in an incorrect timezone (ie. subtract 1 minute from 2002-10-27 1:00 EST and you get 2002-10-27 0:59 EST instead of the correct 2002-10-27 1:59 EDT). A normalize() method is provided to correct this. Unfortunatly these issues cannot be resolved without modifying the Python datetime implementation.

Installation

This package can either be installed from a .egg file using setuptools, or from the tarball using the standard Python distutils.

If you are installing from a tarball, run the following command as an administrative user:

python setup.py install

If you are installing using setuptools, you don’t even need to download anything as the latest version will be downloaded for you from the Python package index:

easy_install --upgrade pytz

If you already have the .egg file, you can use that too:

easy_install pytz-2008g-py2.6.egg

Example & Usage

>>> from datetime import datetime, timedelta
>>> from pytz import timezone
>>> import pytz
>>> utc = pytz.utc
>>> utc.zone
'UTC'
>>> eastern = timezone('US/Eastern')
>>> eastern.zone
'US/Eastern'
>>> amsterdam = timezone('Europe/Amsterdam')
>>> fmt = '%Y-%m-%d %H:%M:%S %Z%z'

This library only supports two ways of building a localized time. The first is to use the .localize() method provided by the pytz library. This is used to localize a naive datetime (datetime with no timezone information):

>>> loc_dt = eastern.localize(datetime(2002, 10, 27, 6, 0, 0))
>>> print loc_dt.strftime(fmt)
2002-10-27 06:00:00 EST-0500

The second way of building a localized time is by converting an existing localized time using the standard .astimezone() method:

>>> ams_dt = loc_dt.astimezone(amsterdam)
>>> ams_dt.strftime(fmt)
'2002-10-27 12:00:00 CET+0100'

Unfortunately using the tzinfo argument of the standard datetime constructors ‘’does not work’’ with pytz for many timezones.

>>> datetime(2002, 10, 27, 12, 0, 0, tzinfo=amsterdam).strftime(fmt)
'2002-10-27 12:00:00 AMT+0020'

It is safe for timezones without daylight savings trasitions though, such as UTC:

>>> datetime(2002, 10, 27, 12, 0, 0, tzinfo=pytz.utc).strftime(fmt)
'2002-10-27 12:00:00 UTC+0000'

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.

>>> utc_dt = datetime(2002, 10, 27, 6, 0, 0, tzinfo=utc)
>>> loc_dt = utc_dt.astimezone(eastern)
>>> loc_dt.strftime(fmt)
'2002-10-27 01:00:00 EST-0500'

This library also allows you to do date arithmetic using local times, although it is more complicated than working in UTC as you need to use the normalize method to handle daylight savings time and other timezone transitions. In this example, loc_dt is set to the instant when daylight savings time ends in the US/Eastern timezone.

>>> before = loc_dt - timedelta(minutes=10)
>>> before.strftime(fmt)
'2002-10-27 00:50:00 EST-0500'
>>> eastern.normalize(before).strftime(fmt)
'2002-10-27 01:50:00 EDT-0400'
>>> after = eastern.normalize(before + timedelta(minutes=20))
>>> after.strftime(fmt)
'2002-10-27 01:10:00 EST-0500'

Creating localtimes is also tricky, and the reason why working with local times is not recommended. Unfortunately, you cannot just pass a ‘tzinfo’ argument when constructing a datetime (see the next section for more details)

>>> dt = datetime(2002, 10, 27, 1, 30, 0)
>>> dt1 = eastern.localize(dt, is_dst=True)
>>> dt1.strftime(fmt)
'2002-10-27 01:30:00 EDT-0400'
>>> dt2 = eastern.localize(dt, is_dst=False)
>>> dt2.strftime(fmt)
'2002-10-27 01:30:00 EST-0500'

Converting between timezones also needs special attention. This also needs to use the normalize method to ensure the conversion is correct.

>>> utc_dt = utc.localize(datetime.utcfromtimestamp(1143408899))
>>> utc_dt.strftime(fmt)
'2006-03-26 21:34:59 UTC+0000'
>>> au_tz = timezone('Australia/Sydney')
>>> au_dt = au_tz.normalize(utc_dt.astimezone(au_tz))
>>> au_dt.strftime(fmt)
'2006-03-27 08:34:59 EST+1100'
>>> utc_dt2 = utc.normalize(au_dt.astimezone(utc))
>>> utc_dt2.strftime(fmt)
'2006-03-26 21:34:59 UTC+0000'

You can take shortcuts when dealing with the UTC side of timezone conversions. Normalize and localize are not really necessary when there are no daylight savings time transitions to deal with.

>>> utc_dt = datetime.utcfromtimestamp(1143408899).replace(tzinfo=utc)
>>> utc_dt.strftime(fmt)
'2006-03-26 21:34:59 UTC+0000'
>>> au_tz = timezone('Australia/Sydney')
>>> au_dt = au_tz.normalize(utc_dt.astimezone(au_tz))
>>> au_dt.strftime(fmt)
'2006-03-27 08:34:59 EST+1100'
>>> utc_dt2 = au_dt.astimezone(utc)
>>> utc_dt2.strftime(fmt)
'2006-03-26 21:34:59 UTC+0000'

Problems with Localtime

The major problem we have to deal with is that certain datetimes may occur twice in a year. For example, in the US/Eastern timezone on the last Sunday morning in October, the following sequence happens:

• 01:00 EDT occurs

• 1 hour later, instead of 2:00am the clock is turned back 1 hour and 01:00 happens again (this time 01:00 EST)

In fact, every instant between 01:00 and 02:00 occurs twice. This means that if you try and create a time in the US/Eastern timezone using the standard datetime syntax, there is no way to specify if you meant before of after the end-of-daylight-savings-time transition.

>>> loc_dt = datetime(2002, 10, 27, 1, 30, 00, tzinfo=eastern)
>>> loc_dt.strftime(fmt)
'2002-10-27 01:30:00 EST-0500'

As you can see, the system has chosen one for you and there is a 50% chance of it being out by one hour. For some applications, this does not matter. However, if you are trying to schedule meetings with people in different timezones or analyze log files it is not acceptable.

The best and simplest solution is to stick with using UTC. The pytz package encourages using UTC for internal timezone representation by including a special UTC implementation based on the standard Python reference implementation in the Python documentation. This timezone unpickles to be the same instance, and pickles to a relatively small size. The UTC implementation can be obtained as pytz.utc, pytz.UTC, or pytz.timezone(‘UTC’).

>>> import pickle, pytz
>>> dt = datetime(2005, 3, 1, 14, 13, 21, tzinfo=utc)
>>> naive = dt.replace(tzinfo=None)
>>> p = pickle.dumps(dt, 1)
>>> naive_p = pickle.dumps(naive, 1)
>>> len(p), len(naive_p), len(p) - len(naive_p)
(60, 43, 17)
>>> new = pickle.loads(p)
>>> new == dt
True
>>> new is dt
False
>>> new.tzinfo is dt.tzinfo
True
>>> pytz.utc is pytz.UTC is pytz.timezone('UTC')
True

Note that this instance is not the same instance (or implementation) as other timezones with the same meaning (GMT, Greenwich, Universal, etc.).

>>> utc is pytz.timezone('GMT')
False

If you insist on working with local times, this library provides a facility for constructing them unambiguously:

>>> loc_dt = datetime(2002, 10, 27, 1, 30, 00)
>>> est_dt = eastern.localize(loc_dt, is_dst=True)
>>> edt_dt = eastern.localize(loc_dt, is_dst=False)
>>> print est_dt.strftime(fmt), '/', edt_dt.strftime(fmt)
2002-10-27 01:30:00 EDT-0400 / 2002-10-27 01:30:00 EST-0500

If you pass None as the is_dst flag to localize(), pytz will refuse to guess and raise exceptions if you try to build ambiguous or non-existent times.

For example, 1:30am on 27th Oct 2002 happened twice in the US/Eastern timezone when the clocks where put back at the end of Daylight Savings Time:

>>> eastern.localize(datetime(2002, 10, 27, 1, 30, 00), is_dst=None)
Traceback (most recent call last):
...
AmbiguousTimeError: 2002-10-27 01:30:00

Similarly, 2:30am on 7th April 2002 never happened at all in the US/Eastern timezone, as the clock where put forward at 2:00am skipping the entire hour:

>>> eastern.localize(datetime(2002, 4, 7, 2, 30, 00), is_dst=None)
Traceback (most recent call last):
...
NonExistentTimeError: 2002-04-07 02:30:00

Both of these exceptions share a common base class to make error handling easier:

>>> isinstance(pytz.AmbiguousTimeError(), pytz.InvalidTimeError)
True
>>> isinstance(pytz.NonExistentTimeError(), pytz.InvalidTimeError)
True

Although localize() handles many cases, it is still not possible to handle all. In cases where countries change their timezone definitions, cases like the end-of-daylight-savings-time occur with no way of resolving the ambiguity. For example, in 1915 Warsaw switched from Warsaw time to Central European time. So at the stroke of midnight on August 5th 1915 the clocks were wound back 24 minutes creating an ambiguous time period that cannot be specified without referring to the timezone abbreviation or the actual UTC offset. In this case midnight happened twice, neither time during a daylight savings time period:

>>> warsaw = pytz.timezone('Europe/Warsaw')
>>> loc_dt1 = warsaw.localize(datetime(1915, 8, 4, 23, 59, 59), is_dst=False)
>>> loc_dt1.strftime(fmt)
'1915-08-04 23:59:59 WMT+0124'
>>> loc_dt2 = warsaw.localize(datetime(1915, 8, 5, 00, 00, 00), is_dst=False)
>>> loc_dt2.strftime(fmt)
'1915-08-05 00:00:00 CET+0100'
>>> str(loc_dt2 - loc_dt1)
'0:24:01'

The only way of creating a time during the missing 24 minutes is converting from another time - because neither of the timezones involved where in daylight savings mode the API simply provides no way to express it:

>>> utc_dt = datetime(1915, 8, 4, 22, 36, tzinfo=pytz.utc)
>>> utc_dt.astimezone(warsaw).strftime(fmt)
'1915-08-04 23:36:00 CET+0100'

The ‘Standard’ Python way of handling all these ambiguities is not to, such as demonstrated in this example using the US/Eastern timezone definition from the Python documentation (Note that this implementation only works for dates between 1987 and 2006 - it is included for tests only!):

>>> from pytz.reference import Eastern # pytz.reference only for tests
>>> dt = datetime(2002, 10, 27, 0, 30, tzinfo=Eastern)
>>> str(dt)
'2002-10-27 00:30:00-04:00'
>>> str(dt + timedelta(hours=1))
'2002-10-27 01:30:00-05:00'
>>> str(dt + timedelta(hours=2))
'2002-10-27 02:30:00-05:00'
>>> str(dt + timedelta(hours=3))
'2002-10-27 03:30:00-05:00'

Notice the first two results? At first glance you might think they are correct, but taking the UTC offset into account you find that they are actually two hours appart instead of the 1 hour we asked for.

>>> from pytz.reference import UTC # pytz.reference only for tests
>>> str(dt.astimezone(UTC))
'2002-10-27 04:30:00+00:00'
>>> str((dt + timedelta(hours=1)).astimezone(UTC))
'2002-10-27 06:30:00+00:00'

Country Information

A mechanism is provided to access the timezones commonly in use for a particular country, looked up using the ISO 3166 country code. It returns a list of strings that can be used to retrieve the relevant tzinfo instance using pytz.timezone():

>>> pytz.country_timezones['nz']
['Pacific/Auckland', 'Pacific/Chatham']

The Olson database comes with a ISO 3166 country code to English country name mapping that pytz exposes as a dictionary:

>>> pytz.country_names['nz']
'New Zealand'

What is UTC

UTC is Universal Time, also known as Greenwich Mean Time or GMT in the United Kingdom. All other timezones are given as offsets from UTC. No daylight savings time occurs in UTC, making it a useful timezone to perform date arithmetic without worrying about the confusion and ambiguities caused by daylight savings time transitions, your country changing its timezone, or mobile computers that move roam through multiple timezones.

Helpers

There are two lists of timezones provided.

all_timezones is the exhaustive list of the timezone names that can be used.

>>> from pytz import all_timezones
>>> len(all_timezones) >= 500
True
>>> 'Etc/Greenwich' in all_timezones
True

common_timezones is a list of useful, current timezones. It doesn’t contain deprecated zones or historical zones, except for a few I’ve deemed in common usage, such as US/Eastern (open a bug report if you think other timezones are deserving of being included here).It is also a sequence of strings.

>>> from pytz import common_timezones
>>> len(common_timezones) < len(all_timezones)
True
>>> 'Etc/Greenwich' in common_timezones
False
>>> 'US/Eastern' in common_timezones
True
>>> 'Australia/Melbourne' in common_timezones
True
>>> 'US/Pacific-New' in all_timezones
True
>>> 'US/Pacific-New' in common_timezones
False

Both common_timezones and all_timezones are alphabetically sorted:

>>> common_timezones_dupe = common_timezones[:]
>>> common_timezones_dupe.sort()
>>> common_timezones == common_timezones_dupe
True
>>> all_timezones_dupe = all_timezones[:]
>>> all_timezones_dupe.sort()
>>> all_timezones == all_timezones_dupe
True

all_timezones and common_timezones are also available as sets.

>>> from pytz import all_timezones_set, common_timezones_set
>>> 'US/Eastern' in all_timezones_set
True
>>> 'US/Eastern' in common_timezones_set
True
>>> 'Australia/Victoria' in common_timezones_set
False

You can also retrieve lists of timezones used by particular countries using the country_timezones() method. It requires an ISO-3166 two letter country code.

>>> from pytz import country_timezones
>>> country_timezones('ch')
['Europe/Zurich']
>>> country_timezones('CH')
['Europe/Zurich']

License

MIT license.

This code is also available as part of Zope 3 under the Zope Public License, Version 2.1 (ZPL).

I’m happy to relicense this code if necessary for inclusion in other open source projects.

Latest Versions

This package will be updated after releases of the Olson timezone database. The latest version can be downloaded from the Python Package Index (PyPI). The code that is used to generate this distribution is hosted on launchpad.net and available using the Bazaar revision control system using:

bzr branch lp:pytz

Bugs, Feature Requests & Patches

Bugs can be reported using Launchpad at https://bugs.launchpad.net/products/pytz

Issues & Limitations

• Offsets from UTC are rounded to the nearest whole minute, so timezones such as Europe/Amsterdam pre 1937 will be up to 30 seconds out. This is a limitation of the Python datetime library.

• If you think a timezone definition is incorrect, I probably can’t fix it. pytz is a direct translation of the Olson timezone database, and changes to the timezone definitions need to be made to this source. If you find errors they should be reported to the time zone mailing list, linked from http://www.twinsun.com/tz/tz-link.htm

Further Reading

More info than you want to know about timezones: http://www.twinsun.com/tz/tz-link.htm

Contact

Stuart Bishop <stuart@stuartbishop.net>