Why Python datetime and JS Date does not match?

Question

I have this code, that returns UTC offset from given date:

>>> import datetime
>>> import pytz
>>> cet = pytz.timezone("Europe/Moscow")
>>> cet.localize(datetime.datetime(2000, 6, 1))
datetime.datetime(2000, 6, 1, 0, 0, tzinfo=<DstTzInfo 'Europe/Moscow' MSD+4:00:00 DST>)
>>> int(cet.localize(datetime.datetime(2000, 6, 1)).utcoffset().seconds/60)
240

Ok, do it in JS using this code ( http://jsfiddle.net/nvn1fef0/ )

new Date(2000, 5, 1).getTimezoneOffset(); // -180

Maybe i doing something wrong? And how i can get plus-minus before offset (like in JS result)?

jfs · Accepted Answer · 2015-07-14 19:50:12Z

On my system both Python and Javascript produce the same result (modulo sign):

>>> from datetime import datetime, timedelta
>>> import pytz
>>> tz = pytz.timezone('Europe/Moscow')
>>> dt = tz.localize(datetime(2000, 6, 1), is_dst=None)
>>> print(dt)
2000-06-01 00:00:00+04:00
>>> dt.utcoffset() // timedelta(minutes=1)
240

And new Date(2000, 6, 1).getTimezoneOffset() returns -240 (different sign, same value).

Python uses: local time = utc time + utc offset definition. While Javascript uses a different definition: utc offset = utc time - local time i.e., both results are correct and have correct signs for the corresponding definitions.

For a portable Javascript solution, you could use momentjs library that provides access to the same tz database as pytz Python module:

> var moscow = moment.tz("2000-06-01", "Europe/Moscow");
undefined
> moscow.format()
"2000-06-01T00:00:00+04:00"

Anand S Kumar · Accepted Answer · 2015-07-15 03:53:47Z

If you print the result for the following -

print(cet.localize(datetime.datetime(2000, 6, 1)).utcoffset())

You will notice that it gives a datetime.timedelta() object which has both days as well as second.

So for timezones that are UTC - <something> , this actually gives days as -1 and then the remaining in seconds. Example -

In [84]: cet = pytz.timezone("America/Los_Angeles")

In [87]: cet.localize(datetime.datetime(2000, 6, 1)).utcoffset()
Out[87]: datetime.timedelta(-1, 61200)

To get the info about the actual offset , you need to use both days as well as seconds , using a code like (For the above timezone - America/Los_Angeles) -

In [88]: int((cet.localize(datetime.datetime(2000, 6, 1)).utcoffset().days*60*60*24 + cet.localize(datetime.datetime(2000, 6, 1)).utcoffset().seconds)/60)
Out[88]: -420

Also, I believe when you are doing - new Date(2000, 5, 1).getTimezoneOffset(); in javascript, I think it is giving you the timezone offset from UTC for today's date, rather than the date 2000/05/01 (Because for the date - 2000/05/01 the correct offset is what you are getting from python - 240 ) . You may checkout TimezoneJS for getting the timezone specific as well as date specific offsets, etc.

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