We Keep Coding

Convert output format of timezone with help of Python

Maybe someone came across and will be able to give advise.
I am getting the time zone "Europe / Moscow", but it needs to be converted to "UTC + 3".
Only an idea with a dictionary comes to mind, but this is rather a last resort.
I would not be surprised if there are ready-made solutions, but I did not google it well)
Example:
timezone = "Europe/Moscow" -> timezone = "UTC+3"

Python 3.9+:
You can use the zoneinfo package. (link)
Make sure to consider daylight saving time.
from datetime import datetime
import zoneinfo
zone = zoneinfo.ZoneInfo("Europe/Moscow")
offset = datetime.now(zone).utcoffset().total_seconds()//(60*60)
print(offset) # 3 / 4 - This depends on daylight saving time
print(f"UTC+{int(offset)}") # UTC+3 / UTC+4
Older:
Try pytz, but make sure to consider daylight saving time. (link)
import pytz
from datetime import datetime
moscow_tz = pytz.timezone("Europe/Moscow")
offset = moscow_tz.utcoffset(datetime.now())
print(offset) # 3:00:00 / 4:00:00 - This depends on daylight saving time
print(f"UTC+{int(offset.total_seconds()//(60*60))}") # UTC+3 / UTC+4

Why does creating a datetime with a tzinfo from pytz show a weird time offset?

Can someone explain me why I do not get the same result in those?
import datetime,pytz
var1 = datetime.datetime(2017,10,25,20,10,50,tzinfo=pytz.timezone("Europe/Athens")))
print(var1)
The output of this code is: 2017-10-25 20:10:50+01:35
import datetime,pytz
var1 = datetime.datetime(2017,10,25,20,10,50)
var1 = pytz.timezone("Europe/Athens").localize(var1)
print(var1)
The output of this code is: 2017-10-25 20:10:50+03:00
My question is why they have different timezones (1:35 and 3:00). I know that the second code is true because my UTC is 3:00. But can you tell me why I am getting 1:35 in the first one?

There is no problem, datetime just happily reports the offset of the tzinfo in whatever reference frame.
By default pytz.timezone doesn't give the UTC offset but the LMT (local mean time) offset:
>>> pytz.timezone("Europe/Athens")
<DstTzInfo 'Europe/Athens' LMT+1:35:00 STD>
# ^^^-------------------- local mean time
However when you localize it:
>>> var1 = datetime.datetime(2017,10,25,20,10,50)
>>> var1 = pytz.timezone("Europe/Athens").localize(var1)
>>> var1.tzinfo
<DstTzInfo 'Europe/Athens' EEST+3:00:00 DST>
# ^^^^-------------------- eastern european summer time
A different offset is now reported, this time based on the EEST.

tzinfo doesn't work well for some timezones and that could be the reason for the wrong result.
pytz doc:
Unfortunately using the tzinfo argument of the standard datetime constructors ‘’does not work’’ with pytz for many timezones.
Using localize or astimezone is a fix to this problem. Doc says that The preferred way of dealing with times is to always work in UTC, converting to localtime only when generating output to be read by humans.
import datetime, pytz
localTimezone = pytz.timezone('Europe/Athens')
var1 = datetime.datetime(2017,10,25,20,10,50,tzinfo=pytz.utc)
loc_dt = var1.astimezone(localTimezone)
fmt = '%Y-%m-%d %H:%M:%S %Z%z'
print(loc_dt.strftime(fmt))
This will print
2017-10-25 23:10:50 EEST+0300

In the second code, you use .localize(), which takes a naive datetime object and interprets it as if it is in that timezone. It does not move the time to another timezone. A naive datetime object has no timezone information to be able to make that move possible.
As you are making the time local in the second code, the time shown in the second one is correct. As you are not making the time local in the first code, the time shown is incorrect.

Elegant way to adjust date timezones in Python

I'm based in the UK, and grappling with summer time BST and timezones.
Here's my code:
TIME_OFFSET = 1 # 0 for GMT, 1 for BST
def RFC3339_to_localHHMM(input):
# Take an XML date (2013-04-08T22:35:00Z)
# return e.g. 08/04 23:35
return (datetime.datetime.strptime(input, '%Y-%m-%dT%H:%M:%SZ') +
datetime.timedelta(hours=TIME_OFFSET)).strftime('%d/%m %H:%M')
Setting a variable like this feels very wrong, but I can't find any elegant way to achieve the above without hideous amounts of code. Am I missing something, and is there no way to (for example) read the system timezone?

To convert UTC to given timezone:
from datetime import datetime
import pytz
local_tz = pytz.timezone("Europe/London") # time zone name from Olson database
def utc_to_local(utc_dt):
return utc_dt.replace(tzinfo=pytz.utc).astimezone(local_tz)
rfc3339s = "2013-04-08T22:35:00Z"
utc_dt = datetime.strptime(rfc3339s, '%Y-%m-%dT%H:%M:%SZ')
local_dt = utc_to_local(utc_dt)
print(local_dt.strftime('%d/%m %H:%M')) # -> 08/04 23:35
See also How to convert a python utc datetime to a local datetime using only python standard library?.

You seem to be asking a few separate questions here.
First, if you only care about your own machine's current local timezone, you don't need to know what it is. Just use the local-to-UTC functions. There are a few holes in the API, but even if you can't find the function you need, you can always just get from local to UTC or vice-versa by going through the POSIX timestamp and the fromtimestamp and utcfromtimestamp methods.
If you want to be able to deal with any timezone, see the top of the docs for the difference between aware and naive objects, but basically: an aware object is one that knows its timezone. So, that's what you need. The problem is that, as the docs say:
Note that no concrete tzinfo classes are supplied by the datetime module. Supporting timezones at whatever level of detail is required is up to the application.
The easiest way to support timezones is to install and use the third-party library pytz.
Meanwhile, as strftime() and strptime() Behavior sort-of explains, strptime always returns a naive object. You then have to call replace and/or astimezone (depending on whether the string was a UTC time or a local time) to get an aware object imbued with the right timezone.
But, even with all this, you still need to know what local timezone you're in, which means you still need a constant. In other words:
TIMEZONE = pytz.timezone('Europe/London')
def RFC3339_to_localHHMM(input):
# Take an XML date (2013-04-08T22:35:00Z)
# return e.g. 08/04 23:35
utc_naive = datetime.datetime.strptime(input, '%Y-%m-%dT%H:%M:%SZ')
utc = utc_naive.replace(pytz.utc)
bst = utc.astimezone(TIMEZONE)
return bst.strftime('%d/%m %H:%M')
So, how do you get the OS to give you the local timezone? Well, that's different for different platforms, and Python has nothing built in to help. But there are a few different third-party libraries that do, such as dateutil. For example:
def RFC3339_to_localHHMM(input):
# Take an XML date (2013-04-08T22:35:00Z)
# return e.g. 08/04 23:35
utc = datetime.datetime.strptime(input, '%Y-%m-%dT%H:%M:%SZ')
bst = utc.astimezone(dateutil.tz.tzlocal())
return bst.strftime('%d/%m %H:%M')
But now we've come full circle. If all you wanted was the local timezone, you didn't really need the timezone at all (at least for your simple use case). So, this is only necessary if you need to support any timezone, and also want to be able to, e.g., default to your local timezone (without having to write two copies of all of your code for the aware and naive cases).
(Also, if you're going to use dateutil in the first place, you might want to use it for more than just getting the timezone—it can basically replacing everything you're doing with both datetime and pytz.)
Of course there are other options besides these libraries—search PyPI, Google, and/or the ActiveState recipes.

If you want to convert a UTC input into a local time, regardless of which timezone you're in, try this:
def utctolocal(input):
if time.localtime()[-1] == 1: st=3600
else: st=0
return time.localtime(time.time()-time.mktime(time.gmtime())+time.mktime(time.localtime(time.mktime(time.strptime(input, '%Y-%m-%dT%H:%M:%SZ'))))+st)
Quite long code, but what it does is it simply adds the difference between time.gmtime() and time.localtime() to the time tuple created from the input.

Here's a function I use to do what I think you want. This assumes that the input is really a gmt, or more precisely, a utc datetime object:
def utc_to_local(utc_dt):
'''Converts a utc datetime obj to local datetime obj.'''
t = utc_dt.timetuple()
secs = calendar.timegm(t)
loc = time.localtime(secs)
return datetime.datetime.fromtimestamp(time.mktime(loc))
Like you said, this relies on the system time zone, which may give you shaky results, as some of the comments have pointed out. It has worked perfectly for me on Windows, however.

A simple function to check if a UCT corresponds to BST in London or GMT (for setting TIME_OFFSET above)
import datetime
def is_BST(input_date):
if input_date.month in range(4,9):
return True
if input_date.month in [11,12,1,2]:
return False
# Find start and end dates for current year
current_year = input_date.year
for day in range(25,32):
if datetime.datetime(current_year,3,day).weekday()==6:
BST_start = datetime.datetime(current_year,3,day,1)
if datetime.datetime(current_year,10,day).weekday()==6:
BST_end = datetime.datetime(current_year,10,day,1)
if (input_date > BST_start) and (input_date < BST_end):
return True
return False

Python datetime subtracting date oddity

I have a datetime object created from which I subtract 13 days as follow:
(date.today()-timedelta(days=13)).strftime('%Y-%m-%d')
The strangeness occurs when I execute the code at 6AM and 8:30AM. At 6AM, the resulting string is returned as (if today is 2012-02-29):
2012-02-15
which is 14 days before the current! However, running the same line at 8:30AM, the resulting string is returned as:
2012-02-16
Which then correct. So far I have not been able to figure out what the difference is between the small period of time. I use timezone naive datetime objects, if that is important. I would like to know what could cause this change in the resulting string date.
Many thanks.
EDIT: (based on eumiro's suggestion below)
datetime.datetime.now() returns:
>>> datetime.datetime(2012, 2, 29, 10, 46, 20, 659862)
And the timezone is Europe/Vienna on the server and in the django application that runs the line of code.
I also tried running a similar line to the one you suggested:
(pytz.timezone(settings.TIME_ZONE).localize(datetime.now(), is_dst=True) - \
timedelta(days=13)).strftime('%Y-%m-%d')
But with the same results... which is why I think I don't think it has much to do with timezones also. But at the same time not sure where else to look.

You live somewhere in America? This is the place where the timezones are around 6-8 hours behind the UTC and that's the time of UTC midnight.
What does datetime.datetime.now() return?
If you want to get the real local time, use this (replace "America/New_York" with your timezone):
from datetime import datetime, timedelta
import pytz
now = datetime.datetime.now(pytz.timezone("America/New_York"))
dt = (now - timedelta(days=13)).strftime('%Y-%m-%d')
and it should return the same correct values from midnight until midnight.

Unfortunately DST is poorly supported in Python.
Even pytz is not perfect, but can be made to work with hacks.
You have to decide what it means to subtract 2 days from 10th, 1p.m., either 2 calendar days or 48 hours or 172800 seconds.

How to get UTC time given timezone?

I am in the US and I want to get the UTC time (without the effect of Daylight Saving Time) for a piece of code:
localtime = strftime("%m%d%y%H%M", gmtime())
Right now that code give me time in Greenwich, England. I know that I have a time offset of -8 (GMT -8). How can I get the UTC time to a specific timezone? (using Python library, not casting the hour to integer, - 8, and than convert it back)

Try the datetime module:
datetime.datetime.utcnow() + datetime.timedelta(hours=-8)
BTW, UTC doesn't have timezones, it's Universal Co-ordinated Time, it's the same everywhere.

Just use the localtime() rather than gmtime() function:
localtime = time.strftime("%m%d%y%H%M", time.localtime())
From the python time module:
"Like gmtime() but converts to local time. If secs is not provided or None, the current time as returned by time() is used. The dst flag is set to 1 when DST applies to the given time."
To see if DST applies:
time.daylight

Supporting timezones at whatever level of detail is required is up to the application. The rules for time adjustment across the world are more political than rational, and there is no standard suitable for every application.
There is a useful third party module for that purpose: http://pytz.sourceforge.net/