Convert a python UTC datetime to a local datetime using only python standard library

Question

I have a python datetime instance that was created using datetime utcnow   and persisted in database   For display  I would like to convert the datetime instance retrieved from the database to local datetime using the default local timezone  i e   as if the datetime was created using datetime now      How can I convert the UTC datetime to a local datetime using only python standard library  e g   no pytz dependency    It seems one solution would be to use datetime astimezone  tz    but how would you get the default local timezone

User · Answer

A simple  but maybe flawed  way that works in Python 2 and 3   import time import datetime  def utc to local dt       return dt - datetime timedelta seconds   time timezone    Its advantage is that it s trivial to write an inverse function

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Here is another way to change timezone in datetime format  I know I wasted my energy on this but I didn t see this page so I don t know how  without min  and sec  cause I don t need it for my project   def change time zone year  month  day  hour         hour   hour   7   lt -- difference       if hour  gt   24          difference   hour - 24         hour   difference         day    1         long months    1  3  5  7  8  10  12          short months    4  6  9  11          if month in short months            if day  gt   30              day   1             month    1             if month  gt  12                year    1         elif month in long months            if day  gt   31              day   1             month    1             if month  gt  12                year    1         elif month    2            if not year 4  0              if day  gt   29                day   1               month    1               if month  gt  12                  year    1           else              if day  gt   28                day   1               month    1               if month  gt  12                  year    1       return datetime int year   int month   int day   int hour   00

User · Answer

In Python 3 3    from datetime import datetime  timezone  def utc to local utc dt       return utc dt replace tzinfo timezone utc  astimezone tz None    In Python 2 3   import calendar from datetime import datetime  timedelta  def utc to local utc dt         get integer timestamp to avoid precision lost     timestamp   calendar timegm utc dt timetuple        local dt   datetime fromtimestamp timestamp      assert utc dt resolution  gt   timedelta microseconds 1      return local dt replace microsecond utc dt microsecond    Using pytz  both Python 2 3    import pytz  local tz   pytz timezone  Europe Moscow     use your local timezone name here   NOTE  pytz reference LocalTimezone   would produce wrong result here     You could use  tzlocal  module to get local timezone on Unix and Win32   from tzlocal import get localzone     pip install tzlocal      get local timezone       local tz   get localzone    def utc to local utc dt       local dt   utc dt replace tzinfo pytz utc  astimezone local tz      return local tz normalize local dt     normalize might be unnecessary   Example  def aslocaltimestr utc dt       return utc to local utc dt  strftime   Y- m- d  H  M  S  f  Z z    print aslocaltimestr datetime 2010   6  6  17  29  7  730000    print aslocaltimestr datetime 2010  12  6  17  29  7  730000    print aslocaltimestr datetime utcnow       Output  Python 3 3  2010-06-06 21 29 07 730000 MSD 0400 2010-12-06 20 29 07 730000 MSK 0300 2012-11-08 14 19 50 093745 MSK 0400   Python 2  2010-06-06 21 29 07 730000  2010-12-06 20 29 07 730000  2012-11-08 14 19 50 093911    pytz  2010-06-06 21 29 07 730000 MSD 0400 2010-12-06 20 29 07 730000 MSK 0300 2012-11-08 14 19 50 146917 MSK 0400   Note  it takes into account DST and the recent change of utc offset for MSK timezone   I don t know whether non-pytz solutions work on Windows

User · Answer

The standard Python library does not come with any tzinfo implementations at all  I ve always considered this a surprising shortcoming of the datetime module   The documentation for the tzinfo class does come with some useful examples  Look for the large code block at the end of the section

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Python 3 9 adds the zoneinfo module so now it can be done as follows  stdlib only    from zoneinfo import ZoneInfo from datetime import datetime  utc unaware   datetime 2020  10  31  12     loaded from database utc aware   utc unaware replace tzinfo ZoneInfo  UTC       make aware local aware   utc aware astimezone ZoneInfo  localtime       convert   Central Europe is 1 or 2 hours ahead of UTC  so local aware is   datetime datetime 2020  10  31  13  0  tzinfo backports zoneinfo ZoneInfo key  localtime      as str   2020-10-31 13 00 00 01 00     Windows has no system time zone database  so here an extra package is needed   pip install tzdata       There is a backport to allow use in Python 3 6 to 3 8   sudo pip install backports zoneinfo   Then   from backports zoneinfo import ZoneInfo

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Building on Alexei s comment  This should work for DST too   import time import datetime  def utc to local dt       if time localtime   tm isdst          return dt - datetime timedelta seconds   time altzone      else          return dt - datetime timedelta seconds   time timezone

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The easiest way I have found is to get the time offset of where you are  then subtract that from the hour   def format time ts offset       if not ts hour  gt   offset          ts   ts replace day ts day-1          ts   ts replace hour ts hour-offset      else          ts   ts replace hour ts hour-offset      return ts   This works for me  in Python 3 5 2

User · Answer

You can t do it with standard library  Using  pytz module you can convert any naive aware datetime object to any other  time zone  Lets see some examples using Python 3      Naive objects created through class method utcnow     To convert a naive object to any other time zone  first you  have to convert  it into aware datetime object  You can use the replace method for converting a naive datetime object to an aware datetime object   Then to convert an aware datetime object to any other timezone you can use astimezone method   The variable pytz all timezones gives you the list of all available time zones in pytz module   import datetime pytz  dtobj1 datetime datetime utcnow      utcnow class method print dtobj1   dtobj3 dtobj1 replace tzinfo pytz UTC   replace method  dtobj hongkong dtobj3 astimezone pytz timezone  Asia Hong Kong     astimezone method print dtobj hongkong       Naive objects created through class method now     Because now method returns current date and time  so you have to make the datetime object timezone aware first  The localize  function  converts a naive datetime object into a timezone-aware datetime object  Then you can use the astimezone method to convert it into another timezone   dtobj2 datetime datetime now    mytimezone pytz timezone  Europe Vienna    my current timezone dtobj4 mytimezone localize dtobj2          localize function  dtobj hongkong dtobj4 astimezone pytz timezone  Asia Hong Kong     astimezone method print dtobj hongkong

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I think I figured it out  computes number of seconds since epoch  then converts to a local timzeone using time localtime  and then converts the time struct back into a datetime     EPOCH DATETIME   datetime datetime 1970 1 1  SECONDS PER DAY   24 60 60  def utc to local datetime  utc datetime        delta   utc datetime - EPOCH DATETIME     utc epoch   SECONDS PER DAY   delta days   delta seconds     time struct   time localtime  utc epoch       dt args   time struct  6     delta microseconds       return datetime datetime   dt args     It applies the summer winter DST correctly    gt  gt  gt  utc to local datetime  datetime datetime 2010  6  6  17  29  7  730000    datetime datetime 2010  6  6  19  29  7  730000   gt  gt  gt  utc to local datetime  datetime datetime 2010  12  6  17  29  7  730000    datetime datetime 2010  12  6  18  29  7  730000

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Use timedelta to switch between timezones  All you need is the offset in hours between timezones  Don t have to fiddle with boundaries for all 6 elements of a datetime object  timedelta handles leap years  leap centuries  etc   too  with ease  You must first    from datetime import datetime  timedelta   Then if offset is the timezone delta in hours      timeout   timein   timedelta hours   offset     where timein and timeout are datetime objects  e g      timein   timedelta hours   -8     converts from GMT to PST   So  how to determine offset  Here is a simple function provided you only have a few possibilities for conversion without using datetime objects that are timezone  aware  which some other answers nicely do  A bit manual  but sometimes clarity is best   def change timezone timein  timezone  timezone out               changes timezone between predefined timezone offsets to GMT     timein - datetime object     timezone -  PST    PDT    GMT   can add more as needed      timezone out -  PST    PDT    GMT   can add more as needed                 simple table lookup             tz offset      PST     GMT   8   PDT   1   PST   0                        GMT     PST   -8   PDT   -7   GMT   0                        PDT     GMT   7   PST   -1   PDT   0       try          offset   tz offset timezone  timezone out      except          msg    Input timezone     timezone     OR output time zone                   timezone out     not recognized          raise DateTimeError msg       return timein   timedelta hours   offset    After looking at the numerous answers and playing around with the tightest code I can think of  for now  it seems best that all applications  where time is important and mixed timezones must be accounted for  should make a real effort to make all datetime objects  aware   Then it would seem the simplest answer is   timeout   timein astimezone pytz timezone  GMT      to convert to GMT for example  Of course  to convert to from any other timezone you wish  local or otherwise  just use the appropriate timezone string that pytz understands  from pytz all timezones   Daylight savings time is then also taken into account

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This is a terrible way to do it but it avoids creating a definition   It fulfills the requirement to stick with the basic Python3 library     Adjust from UST to Eastern Standard Time  dynamic    df my localtime should already be in datetime format  so just in case df  my localtime     pd to datetime df  my localtime    df  my localtime     df  my localtime   dt tz localize  UTC   dt tz convert  America New York   astype str  df  my localtime     pd to datetime df my localtime str  -6

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You can t do it with only the standard library as the standard library doesn t have any timezones  You need pytz or dateutil    gt  gt  gt  from datetime import datetime  gt  gt  gt  now   datetime utcnow    gt  gt  gt  from dateutil import tz  gt  gt  gt  HERE   tz tzlocal    gt  gt  gt  UTC   tz gettz  UTC    The Conversion   gt  gt  gt  gmt   now replace tzinfo UTC   gt  gt  gt  gmt astimezone HERE  datetime datetime 2010  12  30  15  51  22  114668  tzinfo tzlocal      Or well  you can do it without pytz or dateutil by implementing your own timezones  But that would be silly

[python] Convert a python UTC datetime to a local datetime using only python standard library?

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