Following system colour scheme Selected dark colour scheme Selected light colour scheme

Python Enhancement Proposals

PEP 431 – Time zone support improvements

Lennart Regebro <regebro at>
Barry Warsaw <barry at>
Standards Track
11-Dec-2012, 28-Dec-2012, 28-Jan-2013

Table of Contents


This PEP proposes the implementation of concrete time zone support in the Python standard library, and also improvements to the time zone API to deal with ambiguous time specifications during DST changes.


After lengthy discussion it has turned out that the things I thought was problem in datetime’s implementation are intentional. Those include completely ignoring DST transitions when making date time arithmetic. That makes the is_dst flags part of this PEP pointless, as they would have no useful function. datetime by design does not separate between ambiguous datetimes and will never do so.

I therefore withdraw this PEP.

UPDATE: The PEP 615 “Support for the IANA Time Zone Database in the Standard Library” added the zoneinfo module to Python 3.9 and superseded this PEP.


Concrete time zone support

The time zone support in Python has no concrete implementation in the standard library outside of a tzinfo baseclass that supports fixed offsets. To properly support time zones you need to include a database over all time zones, both current and historical, including daylight saving changes. But such information changes frequently, so even if we include the last information in a Python release, that information would be outdated just a few months later.

Time zone support has therefore only been available through two third-party modules, pytz and dateutil, both who include and wrap the “zoneinfo” database. This database, also called “tz” or “The Olsen database”, is the de facto standard time zone database over time zones, and it is included in most Unix and Unix-like operating systems, including OS X.

This gives us the opportunity to include the code that supports the zoneinfo data in the standard library, but by default use the operating system’s copy of the data, which typically will be kept updated by the updating mechanism of the operating system or distribution.

For those who have an operating system that does not include the zoneinfo database, for example Windows, the Python source distribution will include a copy of the zoneinfo database, and a distribution containing the latest zoneinfo database will also be available at the Python Package Index, so it can be easily installed with the Python packaging tools such as easy_install or pip. This could also be done on Unices that are no longer receiving updates and therefore have an outdated database.

With such a mechanism Python would have full-time zone support in the standard library on any platform, and a simple package installation would provide an updated time zone database on those platforms where the zoneinfo database isn’t included, such as Windows, or on platforms where OS updates are no longer provided.

The time zone support will be implemented by making the datetime module into a package, and adding time zone support to datetime based on Stuart Bishop’s pytz module.

Getting the local time zone

On Unix there is no standard way of finding the name of the time zone that is being used. All the information that is available is the time zone abbreviations, such as EST and PDT, but many of those abbreviations are ambiguous and therefore you can’t rely on them to figure out which time zone you are located in.

There is however a standard for finding the compiled time zone information since it’s located in /etc/localtime. Therefore, it is possible to create a local time zone object with the correct time zone information even though you don’t know the name of the time zone. A function in datetime should be provided to return the local time zone.

The support for this will be made by integrating Lennart Regebro’s tzlocal module into the new datetime module.

For Windows it will look up the local Windows time zone name, and use a mapping between Windows time zone names and zoneinfo time zone names provided by the Unicode consortium to convert that to a zoneinfo time zone.

The mapping should be updated before each major or bugfix release, scripts for doing so will be provided in the Tools/ directory.

Ambiguous times

When changing over from daylight savings time (DST) the clock is turned back one hour. This means that the times during that hour happens twice, once with DST and then once without DST. Similarly, when changing to daylight savings time, one hour goes missing.

The current time zone API can not differentiate between the two ambiguous times during a change from DST. For example, in Stockholm the time of 2012-11-28 02:00:00 happens twice, both at UTC 2012-11-28 00:00:00 and also at 2012-11-28 01:00:00.

The current time zone API can not disambiguate this and therefore it’s unclear which time should be returned:

# This could be either 00:00 or 01:00 UTC:
>>> dt = datetime(2012, 10, 28, 2, 0, tzinfo=zoneinfo('Europe/Stockholm'))
# But we can not specify which:
>>> dt.astimezone(zoneinfo('UTC'))
datetime.datetime(2012, 10, 28, 1, 0, tzinfo=<UTC>)

pytz solved this problem by adding is_dst parameters to several methods of the tzinfo objects to make it possible to disambiguate times when this is desired.

This PEP proposes to add these is_dst parameters to the relevant methods of the datetime API, and therefore add this functionality directly to datetime. This is likely the hardest part of this PEP as this involves updating the C version of the datetime library with this functionality, as this involved writing new code, and not just reorganizing existing external libraries.

Implementation API

The zoneinfo database

The latest version of the zoneinfo database should exist in the Lib/tzdata directory of the Python source control system. This copy of the database should be updated before every Python feature and bug-fix release, but not for releases of Python versions that are in security-fix-only-mode.

Scripts to update the database will be provided in Tools/, and the release instructions will be updated to include this update.

New configure options --enable-internal-timezone-database and --disable-internal-timezone-database will be implemented to enable and disable the installation of this database when installing from source. A source install will default to installing them.

Binary installers for systems that have a system-provided zoneinfo database may skip installing the included database since it would never be used for these platforms. For other platforms, for example Windows, binary installers must install the included database.

Changes in the datetime-module

The public API of the new time zone support contains one new class, one new function, one new exception and four new collections. In addition to this, several methods on the datetime object gets a new is_dst parameter.

New class dsttimezone

This class provides a concrete implementation of the tzinfo base class that implements DST support.

New function zoneinfo(name=None, db_path=None)

This function takes a name string that must be a string specifying a valid zoneinfo time zone, i.e. “US/Eastern”, “Europe/Warsaw” or “Etc/GMT”. If not given, the local time zone will be looked up. If an invalid zone name is given, or the local time zone can not be retrieved, the function raises UnknownTimeZoneError.

The function also takes an optional path to the location of the zoneinfo database which should be used. If not specified, the function will look for databases in the following order:

  1. Check if the tzdata-update module is installed, and then use that database.
  2. Use the database in /usr/share/zoneinfo, if it exists.
  3. Use the Python-provided database in Lib/tzdata.

If no database is found an UnknownTimeZoneError or subclass thereof will be raised with a message explaining that no zoneinfo database can be found, but that you can install one with the tzdata-update package.

New parameter is_dst

A new is_dst parameter is added to several methods to handle time ambiguity during DST changeovers.

  • tzinfo.utcoffset(dt, is_dst=False)
  • tzinfo.dst(dt, is_dst=False)
  • tzinfo.tzname(dt, is_dst=False)
  • datetime.astimezone(tz, is_dst=False)

The is_dst parameter can be False (default), True, or None.

False will specify that the given datetime should be interpreted as not happening during daylight savings time, i.e. that the time specified is after the change from DST. This is default to preserve existing behavior.

True will specify that the given datetime should be interpreted as happening during daylight savings time, i.e. that the time specified is before the change from DST.

None will raise an AmbiguousTimeError exception if the time specified was during a DST change over. It will also raise a NonExistentTimeError if a time is specified during the “missing time” in a change to DST.

New exceptions

  • UnknownTimeZoneError

    This exception is a subclass of KeyError and raised when giving a time zone specification that can’t be found:

    >>> datetime.zoneinfo('Europe/New_York')
    Traceback (most recent call last):
    UnknownTimeZoneError: There is no time zone called 'Europe/New_York'
  • InvalidTimeError

    This exception serves as a base for AmbiguousTimeError and NonExistentTimeError, to enable you to trap these two separately. It will subclass from ValueError, so that you can catch these errors together with inputs like the 29th of February 2011.

  • AmbiguousTimeError

    This exception is raised when giving a datetime specification that is ambiguous while setting is_dst to None:

    >>> datetime(2012, 11, 28, 2, 0, tzinfo=zoneinfo('Europe/Stockholm'), is_dst=None)
    Traceback (most recent call last):
    AmbiguousTimeError: 2012-10-28 02:00:00 is ambiguous in time zone Europe/Stockholm
  • NonExistentTimeError

    This exception is raised when giving a datetime specification for a time that due to daylight saving does not exist, while setting is_dst to None:

    >>> datetime(2012, 3, 25, 2, 0, tzinfo=zoneinfo('Europe/Stockholm'), is_dst=None)
    Traceback (most recent call last):
    NonExistentTimeError: 2012-03-25 02:00:00 does not exist in time zone Europe/Stockholm

New collections

  • all_timezones is the exhaustive list of the time zone names that can be used, listed alphabetically.
  • common_timezones is a list of useful, current time zones, listed alphabetically.

The tzdata-update-package

The zoneinfo database will be packaged for easy installation with easy_install/pip/buildout. This package will not install any Python code, and will not contain any Python code except that which is needed for installation.

It will be kept updated with the same tools as the internal database, but released whenever the zoneinfo-database is updated, and use the same version schema.

Differences from the pytz API

  • pytz has the functions localize() and normalize() to work around that tzinfo doesn’t have is_dst. When is_dst is implemented directly in datetime.tzinfo they are no longer needed.
  • The timezone() function is called zoneinfo() to avoid clashing with the timezone class introduced in Python 3.2.
  • zoneinfo() will return the local time zone if called without arguments.
  • The class pytz.StaticTzInfo is there to provide the is_dst support for static time zones. When is_dst support is included in datetime.tzinfo it is no longer needed.
  • InvalidTimeError subclasses from ValueError.



Last modified: 2023-09-09 17:39:29 GMT