Convert UTC datetime string to local datetime

Question

I ve never had to convert time to and from UTC  Recently had a request to have my app be timezone aware  and I ve been running myself in circles  Lots of information on converting local time to UTC  which I found fairly elementary  maybe I m doing that wrong as well   but I can not find any information on easily converting the UTC time to the end-users timezone   In a nutshell  and android app sends me  appengine app  data and within that data is a timestamp  To store that timestamp to utc time I am using   datetime utcfromtimestamp timestamp    That seems to be working  When my app stores the data  it is being store as 5 hours ahead  I am EST -5   The data is being stored on appengine s BigTable  and when retrieved it comes out as a string like so     2011-01-21 02 37 21    How do I convert this string to a DateTime in the users correct time zone   Also  what is the recommended storage for a users timezone information    How do you typically store tz info ie   -5 00  or  EST  etc etc    I m sure the answer to my first question might contain a parameter the answers the second

User · Answer

You can use arrow  from datetime import datetime import arrow  now   datetime utcnow    print arrow get now  to  local   format       2018-04-04 15 59 24 02 00    you can feed arrow get   with anything  timestamp  iso string etc

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I traditionally defer this to the frontend -- send times from the backend as timestamps or some other datetime format in UTC  then let the client figure out the timezone offset and render this data in the proper timezone   For a webapp  this is pretty easy to do in javascript -- you can figure out the browser s timezone offset pretty easily using builtin methods and then render the data from the backend properly

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Here s a resilient method that doesn t depend on any external libraries   from datetime import datetime import time  def datetime from utc to local utc datetime       now timestamp   time time       offset   datetime fromtimestamp now timestamp  - datetime utcfromtimestamp now timestamp      return utc datetime   offset   This avoids the timing issues in DelboyJay s example  And the lesser timing issues in Erik van Oosten s amendment   As an interesting footnote  the timezone offset computed above can differ from the following seemingly equivalent expression  probably due to daylight savings rule changes   offset   datetime fromtimestamp 0  - datetime utcfromtimestamp 0    NO    Update  This snippet has the weakness of using the UTC offset of the present time  which may differ from the UTC offset of the input datetime  See comments on this answer for another solution   To get around the different times  grab the epoch time from the time passed in   Here s what I do   def utc2local  utc       epoch   time mktime utc timetuple        offset   datetime fromtimestamp  epoch  - datetime utcfromtimestamp  epoch      return utc   offset

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This answer should be helpful if you don t want to use any other modules besides datetime   datetime utcfromtimestamp timestamp  returns a naive datetime object  not an aware one   Aware ones are timezone aware  and naive are not  You want an aware one if you want to convert between timezones  e g  between UTC and local time    If you aren t the one instantiating the date to start with  but you can still create a naive datetime object in UTC time  you might want to try this Python 3 x code to convert it   import datetime  d datetime datetime strptime  2011-01-21 02 37 21     Y- m- d  H  M  S    Get your naive datetime object d d replace tzinfo datetime timezone utc   Convert it to an aware datetime object in UTC time  d d astimezone    Convert it to your local timezone  still aware  print d strftime   d  b  Y   I  M  S  f  p   Z     Print it with a directive of choice   Be careful not to mistakenly assume that if your timezone is currently MDT that daylight savings doesn t work with the above code since it prints MST  You ll note that if you change the month to August  it ll print MDT   Another easy way to get an aware datetime object  also in Python 3 x  is to create it with a timezone specified to start with  Here s an example  using UTC   import datetime  sys  aware utc dt obj datetime datetime now datetime timezone utc   create an aware datetime object dt obj local aware utc dt obj astimezone    convert it to local time   The following section is just code for a directive I made that I liked  if sys platform   win32       directive    d  b  Y    I  M  S  f  p   Z  else      directive   -d  b  Y   -I  M  S  f  p   Z   print dt obj local strftime directive     If you use Python 2 x  you ll probably have to subclass datetime tzinfo and use that to help you create an aware datetime object  since datetime timezone doesn t exist in Python 2 x

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You can use calendar timegm to convert your time to seconds since Unix epoch and time localtime to convert back   import calendar import time  time tuple   time strptime  2011-01-21 02 37 21     Y- m- d  H  M  S   t   calendar timegm time tuple   print time ctime t    Gives Fri Jan 21 05 37 21 2011  because I m in UTC 03 00 timezone

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If you want to get the correct result even for the time that corresponds to an ambiguous local time  e g   during a DST transition  and or the local utc offset is different at different times in your local time zone then use pytz timezones      usr bin env python from datetime import datetime import pytz        pip install pytz import tzlocal     pip install tzlocal  local timezone   tzlocal get localzone     get pytz tzinfo utc time   datetime strptime  2011-01-21 02 37 21     Y- m- d  H  M  S   local time   utc time replace tzinfo pytz utc  astimezone local timezone

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If you don t want to provide your own tzinfo objects  check out the python-dateutil library   It provides tzinfo implementations on top of a zoneinfo  Olson  database such that you can refer to time zone rules by a somewhat canonical name   from datetime import datetime from dateutil import tz    METHOD 1  Hardcode zones  from zone   tz gettz  UTC   to zone   tz gettz  America New York      METHOD 2  Auto-detect zones  from zone   tz tzutc   to zone   tz tzlocal      utc   datetime utcnow   utc   datetime strptime  2011-01-21 02 37 21     Y- m- d  H  M  S      Tell the datetime object that it s in UTC time zone since    datetime objects are  naive  by default utc   utc replace tzinfo from zone     Convert time zone central   utc astimezone to zone    Edit Expanded example to show strptime usage  Edit 2 Fixed API usage to show better entry point method  Edit 3 Included auto-detect methods for timezones  Yarin

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import datetime  def utc str to local str utc str  str  utc format  str  local format  str                param utc str  UTC time string      param utc format  format of UTC time string      param local format  format of local time string      return  local time string             temp1   datetime datetime strptime utc str  utc format      temp2   temp1 replace tzinfo datetime timezone utc      local time   temp2 astimezone       return local time strftime local format   utc    2018-10-17T00 00 00 111Z  utc fmt     Y- m- dT H  M  S  fZ  local fmt     Y- m- dT H  M  S 08 00  local string   utc str to local str utc  utc fmt  local fmt  print local string      2018-10-17T08 00 00 08 00   for example  my timezone is   08 00   input utc   2018-10-17T00 00 00 111Z  then I will get output   2018-10-17T08 00 00 08 00

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Consolidating the answer from franksands into a convenient method   import calendar import datetime  def to local datetime utc dt               convert from utc datetime to a locally aware datetime according to the host timezone       param utc dt  utc datetime      return  local timezone datetime             return datetime datetime fromtimestamp calendar timegm utc dt timetuple

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See the datetime documentation on tzinfo objects   You have to implement the timezones you want to support yourself   The are examples at the bottom of the documentation   Here s a simple example   from datetime import datetime tzinfo timedelta  class Zone tzinfo       def   init   self offset isdst name           self offset   offset         self isdst   isdst         self name   name     def utcoffset self  dt           return timedelta hours self offset    self dst dt      def dst self  dt               return timedelta hours 1  if self isdst else timedelta 0      def tzname self dt            return self name  GMT   Zone 0 False  GMT   EST   Zone -5 False  EST    print datetime utcnow   strftime   m  d  Y  H  M  S  Z   print datetime now GMT  strftime   m  d  Y  H  M  S  Z   print datetime now EST  strftime   m  d  Y  H  M  S  Z    t   datetime strptime  2011-01-21 02 37 21    Y- m- d  H  M  S   t   t replace tzinfo GMT  print t print t astimezone EST    Output  01 22 2011 21 52 09  01 22 2011 21 52 09 GMT 01 22 2011 16 52 09 EST 2011-01-21 02 37 21 00 00 2011-01-20 21 37 21-05 00a

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If using Django  you can use the timezone localtime method   from django utils import timezone date    datetime datetime 2014  8  1  20  15  0  513000  tzinfo  lt UTC gt    timezone localtime date    datetime datetime 2014  8  1  16  15  0  513000  tzinfo  lt DstTzInfo  America New York  EDT-1 day  20 00 00 DST gt

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Here is a quick and dirty version that uses the local systems settings to work out the time difference  NOTE  This will not work if you need to convert to a timezone that your current system is not running in  I have tested this with UK settings under BST timezone  from datetime import datetime def ConvertP4DateTimeToLocal timestampValue      assert isinstance timestampValue  int        get the UTC time from the timestamp integer value     d   datetime utcfromtimestamp  timestampValue         calculate time difference from utcnow and the local system time reported by OS    offset   datetime now   - datetime utcnow         Add offset to UTC time and return it    return d   offset

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From the answer here  you can use the time module to convert from utc to the local time set in your computer   utc time   time strptime  2018-12-13T10 32 00 000     Y- m- dT H  M  S  f   utc seconds   calendar timegm utc time  local time   time localtime utc seconds

[python] Convert UTC datetime string to local datetime

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