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Python – Convert UTC datetime string to local datetime

Posted by: admin November 1, 2017 Leave a comment

Questions:

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.

Answers:

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)

Questions:
Answers:

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

Questions:
Answers:

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

Questions:
Answers:

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)

Questions:
Answers:

If using django, you can use the timezone.localtime method (see https://docs.djangoproject.com/en/dev/topics/i18n/timezones/).

from django.utils import timezone
date 
# datetime.datetime(2014, 8, 1, 20, 15, 0, 513000, tzinfo=<UTC>)

timezone.localtime(date)
# datetime.datetime(2014, 8, 1, 16, 15, 0, 513000, tzinfo=<DstTzInfo 'America/New_York' EDT-1 day, 20:00:00 DST>)

Questions:
Answers:

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.

Questions:
Answers:

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

Questions:
Answers:

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).

Questions:
Answers:

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.