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Convert timezone naive datetime string to UTC datetime (but not using pytz)

We have the need to convert an incoming datetime string into a UTC datetime string. The incoming string is in this format:
'01/28/2022 12:00 AM'
with intended output as:
'2022-01-28T08:00:00Z'
The string has no declared timezone but is supposed to be PST. We'd like to code this in a way that allows for multiple timezones and date string formats. We can get this working using the following code:
from datetime import datetime
import pytz
UTC_TIME_FORMAT = '%Y-%m-%dT%H:%M:%SZ'
PROVIDER_DATETIME_FORMAT = '%m/%d/%Y %I:%M %p'
PROVIDER_TIMEZONE = 'America/Los_Angeles'
def dt_string_to_utc_converter(datetime_str)
utc_timezone = pytz.utc
pst_timezone = pytz.timezone(PROVIDER_TIMEZONE)
naive_date = datetime.strptime(datetime_str, PROVIDER_DATETIME_FORMAT)
pst_date = pst_timezone.localize(naive_date)
utc_date = pst_date.astimezone(utc_timezone)
utc_date_str = utc_date.strftime(UTC_TIME_FORMAT)
return utc_date_str
However (for reasons I won't get into) we have been asked to not use the pytz library and instead stick to Python's native datetime, and dateutil libraries. Currently we are committed to Python 3.8.
I've tried a number of things things using datetime's strptime and dateutil's parser, but all to dead ends so far. Are there any obvious patterns i am missing?
Thanks

for the record, here's an option using Python 3.9's zoneinfo and Python 3.7's isoformat.
from datetime import datetime
from zoneinfo import ZoneInfo
def to_isoformat(s, tzname, inputformat='%m/%d/%Y %I:%M %p'):
"""
convert an input string "s" representing date/time in format "inputformat" in
time zone "tzname" to UTC and return an ISO format string with Z denoting UTC.
Parameters
----------
s : str
string representing date/time.
tzname : str
IANA time zone name of time zone that s belongs in.
inputformat : str, optional
date/time string format. The default is '%m/%d/%Y %I:%M %p'.
Returns
-------
str
ISO format string, with seconds precision and Z denoting UTC.
"""
# to datetime and set time zone
dt = datetime.strptime(s, inputformat).replace(tzinfo=ZoneInfo(tzname))
# convert to UTC and make ISO format with Z for UTC
return dt.astimezone(ZoneInfo("UTC")).isoformat(timespec='seconds').replace('+00:00', 'Z')
print(to_isoformat('01/28/2022 12:00 AM', 'America/Los_Angeles'))
# 2022-01-28T08:00:00Z
With Python 3.8, you can basically just replace from zoneinfo import ZoneInfo with from dateutil.tz import gettz as ZoneInfo.

Well, after some more digging I managed to find refs here and here that laid out the basics of what I needed. Final code looks like this:
from datetime import datetime
from dateutil import tz, parser, utils
UTC_TIME_FORMAT = '%Y-%m-%dT%H:%M:%SZ'
PROVIDER_TIMEZONE = 'America/Los_Angeles'
def dt_string_to_utc_converter(datetime_str)
parsed_dt = parser.parse(datetime_str)
localized_dt = utils.default_tzinfo(parsed_dt, tz.gettz(PROVIDER_TIMEZONE))
utc_dt = localized_dt.astimezone(tz.tzutc())
utc_dt_str = utc_dt.strftime(UTC_TIME_FORMAT)

Python convert timestamp to unix

I know these questions have been asked before but I'm struggling to convert a timestamp string to a unix time and figuring out whether the datetime objects are naive or aware
For example, to convert the time "2021-05-19 12:51:47" to unix:
>>> from datetime import datetime as dt
>>> dt_obj = dt.strptime("2021-05-19 12:51:47", "%Y-%m-%d %H:%M:%S")
>>> dt_obj
datetime.datetime(2021, 5, 19, 12, 51, 47)
is dt_obj naive or aware and how would you determine this? The methods on dt_obj such as timetz, tzinfo, and tzname don't seem to indicate anything - does that mean that dt_obj is naive?
Then to get unix:
>>> dt_obj.timestamp()
1621421507.0
However when I check 1621421507.0 on say https://www.unixtimestamp.com then it tells me that gmt for the above is Wed May 19 2021 10:51:47 GMT+0000, ie 2 hours behind the original timestamp?

since Python's datetime treats naive datetime as local time by default, you need to set the time zone (tzinfo attribute):
from datetime import datetime, timezone
# assuming "2021-05-19 12:51:47" represents UTC:
dt_obj = datetime.fromisoformat("2021-05-19 12:51:47").replace(tzinfo=timezone.utc)
Or, as #Wolf suggested, instead of setting the tzinfo attribute explicitly, you can also modify the input string by adding "+00:00" which is parsed to UTC;
dt_obj = datetime.fromisoformat("2021-05-19 12:51:47" + "+00:00")
In any case, the result
dt_obj.timestamp()
# 1621428707.0
now converts as expected on https://www.unixtimestamp.com/:

As long as you don't specify the timezone when calling strptime, you will produce naive datetime objects. You may pass time zone information via %z format specifier and +00:00 added to the textual date-time representation to get a timezone aware datetime object:
from datetime import datetime
dt_str = "2021-05-19 12:51:47"
print(dt_str)
dt_obj = datetime.strptime(dt_str+"+00:00", "%Y-%m-%d %H:%M:%S%z")
print(dt_obj)
print(dt_obj.timestamp())
The of above script is this:
2021-05-19 12:51:47
2021-05-19 12:51:47+00:00
1621428707.0

datetime.timestamp()
Naive datetime instances are assumed to represent local time and this method relies on the platform C mktime() function to perform the conversion.
So using this does automatically apply yours machine current timezone, following recipe is given to calculate timestamp from naive datetime without influence of timezone:
timestamp = (dt - datetime(1970, 1, 1)) / timedelta(seconds=1)

'Europe/Madrid' timezone doesn't match 'Etc/GMT+1'

I'm trying to convert a UTC timestamp to one in the Spanish timezone.
>>> import datetime as dt
>>> import pytz
>>> today = dt.datetime.utcfromtimestamp(1573516800)
datetime.datetime(2019, 11, 12, 0, 0)
>>> today.replace(tzinfo=pytz.timezone('Europe/Madrid')).timestamp()
1573517700.0
>>> today.replace(tzinfo=pytz.timezone('Etc/GMT+1')).timestamp()
1573520400.0
I'm surprised that I get different results for Europe/Madrid and Etc/GMT+1. Why is this? Should Europe/Madrid be used differently, or it is possibly a bug?

A few things:
Europe/Madrid is UTC+1 during standard time, and UTC+2 during summer time (aka daylight saving time).
Etc/GMT+1 is UTC-1 for the entire year. Note the sign is opposite what you might expect. See the explanation in the tzdata sources, and on Wikipedia.
Since Madrid is on UTC+1 on the date you gave, you would get the same result for that date if you used Etc/GMT-1. However, I don't recommend that, as you would then later get the wrong result for a date during summer time.
The Etc/GMT±X zones are intended to be used primarily for non-localizable scenarios such as tracking time onboard ships at sea - not for populated locations on land.
As Mason's answer showed, you should be using the localize function rather than replace to assign a time zone. This is covered in the pytz documentation.

UTC Timestamp: The number of seconds since January 1st, 1970 at UTC.
Python datetime: A nice way of seeing this time that is user friendly
The UTC timestamp is not effected by timezones, but the datetime is.
This code takes the given timestamp and converts it to a UTC datetime and a Europe/Madrid timezone.
import datetime as dt
import pytz
# define the old and new timezones
old_timezone = pytz.timezone("UTC")
new_timezone = pytz.timezone("Europe/Madrid")
# get an 'offset-aware' datetime
today = dt.datetime.utcfromtimestamp(1573516800)
my_datetime = old_timezone.localize(today)
# returns datetime in the new timezone
my_datetime_in_new_timezone = my_datetime.astimezone(new_timezone)
print("Old:", str(my_datetime), "\nNew:", str(my_datetime_in_new_timezone), "\nDifference:",
str(my_datetime - my_datetime_in_new_timezone))
Output:
Old: 2019-11-12 00:00:00+00:00
New: 2019-11-12 01:00:00+01:00
Difference: 0:00:00
Code adapted from:
Python: How do you convert datetime/timestamp from one timezone to another timezone?

Python time library: how do I preserve dst with strptime and strftime

I need to store a timestamp in a readable format, and then later on I need to convert it to epoch for comparison purposes.
I tried doing this:
import time
format = '%Y %m %d %H:%M:%S +0000'
timestamp1 = time.strftime(format,time.gmtime()) # '2016 03 25 04:06:22 +0000'
t1 = time.strptime(timestamp1, format) # time.struct_time(..., tm_isdst=-1)
time.sleep(1)
epoch_now = time.mktime(time.gmtime())
epoch_t1 = time.mktime(t1)
print "Delta: %s" % (epoch_now - epoch_t1)
Running this, instead of getting Delta of 1 sec, I get 3601 (1 hr 1 sec), CONSISTENTLY.
Investigating further, it seems that when I just do time.gmtime(), the struct has tm_isdst=0, whereas the converted struct t1 from timestamp1 string has tm_isdst=-1.
How can I ensure the isdst is preserved to 0. I think that's probably the issue here.
Or is there a better way to record time in human readable format (UTC), and yet be able to convert back to epoch properly for time diff calculation?
UPDATES:
After doing more research last night, I switched to using datetime because it preserves more information in the datetime object, and this is confirmed by albertoql answer below.
Here's what I have now:
from datetime import datetime
format = '%Y-%m-%d %H:%M:%S.%f +0000' # +0000 is optional; only for user to see it's UTC
d1 = datetime.utcnow()
timestamp1 = d1.strftime(format)
d1a = datetime.strptime(timestamp1, format)
time.sleep(1)
d2 = datetime.utcnow()
print "Delta: %s" % (d2 - d1a).seconds
I chose not to add tz to keep it simple/shorter; I can still strptime that way.

Below, first an explanation about the problem, then two possible solutions, one using time, another using datetime.
Problem explanation
The problem is on the observation that the OP made in the question: tm_isdst=-1. tm_isdst is a flag that determines whether daylight savings time is in effect or not (see for more details https://docs.python.org/2/library/time.html#time.struct_time).
Specifically, given the format of the string for the time from the OP (that complies with RFC 2822 Internet email standard), [time.strptime]4 does not store the information about the timezone, namely +0000. Thus, when the struct_time is created again according to the information in the string, tm_isdst=-1, namely unknown. The guess on how to fill in that information when making the calculation is based on the local system. For example, as if the system refers to North America, where daylight savings time is in effect, tm_isdst is set.
Solution with time
If you want to use only time package, then, the easiest way to parse directly the information is to specify that the time is in UTC, and thus adding %Z to the format. Note that time does not provide a way to store the information about the timezone in struct_time. As a result, it does not print the actual time zone associated with the time saved in the variable. The time zone is retrieved from the system. Therefore, it is not possible to directly use the same format for time.strftime. The part of the code for writing and reading the string would look like:
format = '%Y %m %d %H:%M:%S UTC'
format2 = '%Y %m %d %H:%M:%S %Z'
timestamp1 = time.strftime(format, time.gmtime())
t1 = time.strptime(timestamp1, format2)
Solution with datetime
Another solution involves the use datetime and dateutil packages, which directly support timezone, and the code could be (assuming that preserving the timezone information is a requirement):
from datetime import datetime
from dateutil import tz, parser
import time
time_format = '%Y %m %d %H:%M:%S %z'
utc_zone = tz.gettz('UTC')
utc_time1 = datetime.utcnow()
utc_time1 = utc_time1.replace(tzinfo=utc_zone)
utc_time1_string = utc_time1.strftime(time_format)
utc_time1 = parser.parse(utc_time1_string)
time.sleep(1)
utc_time2 = datetime.utcnow()
utc_time2 = utc_time2.replace(tzinfo=utc_zone)
print "Delta: %s" % (utc_time2 - utc_time1).total_seconds()
There are some aspects to pay attention to:
After the call of utcnow, the timezone is not set, as it is a naive UTC datetime. If the information about UTC is not needed, it is possible to delete both lines where the timezone is set for the two times, and the result would be the same, as there is no guess about DST.
It is not possible to use datetime.strptime because of %z, which is not correctly parsed. If the string contains the information about the timezone, then parser should be used.
It is possible to directly perform the difference from two instances of datetime and transform the resulting delta into seconds.
If it is necessary to get the time in seconds since the epoch, an explicit computation should be made, as there is no direct function that does that automatically in datetime (at the time of the answer). Below the code, for example for utc_time2:
epoch_time = datetime(1970,1,1)
epoch2 = (utc_time2 - epoch_time).total_seconds()
datetime.resolution, namely the smallest possible difference between two non-equal datetime objects. This results in a difference that is up to the resolution.

Convert UTC datetime string to local datetime

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.

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)

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

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

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)

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.

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=<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>)

The following worked for me in a Cloud environment for US west:
import datetime
import pytz
#set the timezone
tzInfo = pytz.timezone('America/Los_Angeles')
dt = datetime.datetime.now(tz=tzInfo)
print(dt)

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()))

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

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).

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_tz_example_str = '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'
# call my function here
local_tz_str = utc_str_to_local_str(utc_tz_example_str, utc_fmt, local_fmt)
print(local_tz_str) # 2018-10-17T08:00:00+08:00
When I input utc_tz_example_str = 2018-10-17T00:00:00.111Z, （UTC +00:00）
then I will get local_tz_str = 2018-10-17T08:00:00+08:00 （My target timezone +08:00）
parameter utc_format is a format determined by your specific utc_tz_example_str.
parameter local_fmt is the final desired format.
In my case, my desired format is %Y-%m-%dT%H:%M:%S+08:00 ( +08:00 timezone). You should construct the format you want.

This worked for me:
from django.utils import timezone
from datetime import timedelta,datetime
ist_time = timezone.now() + timedelta(hours=5,minutes=30)
#second method
ist_time = datetime.now() + timedelta(hours=5,minutes=30)

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.

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)

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

Short and simple:
from datetime import datetime
t = "2011-01-21 02:37:21"
datetime.fromisoformat(t) + (datetime.now() - datetime.utcnow())