How to convert local time string to UTC?

StackOverflow https://stackoverflow.com/questions/79797

Question

How do I convert a datetime string in local time to a string in UTC time?

I'm sure I've done this before, but can't find it and SO will hopefully help me (and others) do that in future.

Clarification: For example, if I have 2008-09-17 14:02:00 in my local timezone (+10), I'd like to generate a string with the equivalent UTC time: 2008-09-17 04:02:00.

Also, from http://lucumr.pocoo.org/2011/7/15/eppur-si-muove/, note that in general this isn't possible as with DST and other issues there is no unique conversion from local time to UTC time.

Was it helpful?

Solution 3

Thanks @rofly, the full conversion from string to string is as follows:

time.strftime("%Y-%m-%d %H:%M:%S", 
              time.gmtime(time.mktime(time.strptime("2008-09-17 14:04:00", 
                                                    "%Y-%m-%d %H:%M:%S"))))

My summary of the time/calendar functions:

time.strptime
string --> tuple (no timezone applied, so matches string)

time.mktime
local time tuple --> seconds since epoch (always local time)

time.gmtime
seconds since epoch --> tuple in UTC

and

calendar.timegm
tuple in UTC --> seconds since epoch

time.localtime
seconds since epoch --> tuple in local timezone

OTHER TIPS

First, parse the string into a naive datetime object. This is an instance of datetime.datetime with no attached timezone information. See documentation for datetime.strptime for information on parsing the date string.

Use the pytz module, which comes with a full list of time zones + UTC. Figure out what the local timezone is, construct a timezone object from it, and manipulate and attach it to the naive datetime.

Finally, use datetime.astimezone() method to convert the datetime to UTC.

Source code, using local timezone "America/Los_Angeles", for the string "2001-2-3 10:11:12":

import pytz, datetime
local = pytz.timezone ("America/Los_Angeles")
naive = datetime.datetime.strptime ("2001-2-3 10:11:12", "%Y-%m-%d %H:%M:%S")
local_dt = local.localize(naive, is_dst=None)
utc_dt = local_dt.astimezone(pytz.utc)

From there, you can use the strftime() method to format the UTC datetime as needed:

utc_dt.strftime ("%Y-%m-%d %H:%M:%S")

The datetime module's utcnow() function can be used to obtain the current UTC time.

>>> import datetime
>>> utc_datetime = datetime.datetime.utcnow()
>>> utc_datetime.strftime("%Y-%m-%d %H:%M:%S")
'2010-02-01 06:59:19'

As the link mentioned above by Tom: http://lucumr.pocoo.org/2011/7/15/eppur-si-muove/ says:

UTC is a timezone without daylight saving time and still a timezone without configuration changes in the past.

Always measure and store time in UTC.

If you need to record where the time was taken, store that separately. Do not store the local time + timezone information!

NOTE - If any of your data is in a region that uses DST, use pytz and take a look at John Millikin's answer.

If you want to obtain the UTC time from a given string and your lucky enough to be in a region in the world that either doesn't use DST, or you have data that is only offset from UTC without DST applied:

--> using local time as the basis for the offset value:

>>> # Obtain the UTC Offset for the current system:
>>> UTC_OFFSET_TIMEDELTA = datetime.datetime.utcnow() - datetime.datetime.now()
>>> local_datetime = datetime.datetime.strptime("2008-09-17 14:04:00", "%Y-%m-%d %H:%M:%S")
>>> result_utc_datetime = local_datetime + UTC_OFFSET_TIMEDELTA
>>> result_utc_datetime.strftime("%Y-%m-%d %H:%M:%S")
'2008-09-17 04:04:00'

--> Or, from a known offset, using datetime.timedelta():

>>> UTC_OFFSET = 10
>>> result_utc_datetime = local_datetime - datetime.timedelta(hours=UTC_OFFSET)
>>> result_utc_datetime.strftime("%Y-%m-%d %H:%M:%S")
'2008-09-17 04:04:00'

UPDATE:

Since python 3.2 datetime.timezone is available. You can generate a timezone aware datetime object with the command below:

import datetime

timezone_aware_dt = datetime.datetime.now(datetime.timezone.utc)

If your ready to take on timezone conversions go read this:

https://medium.com/@eleroy/10-things-you-need-to-know-about-date-and-time-in-python-with-datetime-pytz-dateutil-timedelta-309bfbafb3f7

Here's a summary of common Python time conversions.

Some methods drop fractions of seconds, and are marked with (s). An explicit formula such as ts = (d - epoch) / unit can be used instead (thanks jfs).

  • struct_time (UTC) → POSIX (s):
    calendar.timegm(struct_time)
  • Naïve datetime (local) → POSIX (s):
    calendar.timegm(stz.localize(dt, is_dst=None).utctimetuple())
    (exception during DST transitions, see comment from jfs)
  • Naïve datetime (UTC) → POSIX (s):
    calendar.timegm(dt.utctimetuple())
  • Aware datetime → POSIX (s):
    calendar.timegm(dt.utctimetuple())
  • POSIX → struct_time (UTC, s):
    time.gmtime(t)
    (see comment from jfs)
  • Naïve datetime (local) → struct_time (UTC, s):
    stz.localize(dt, is_dst=None).utctimetuple()
    (exception during DST transitions, see comment from jfs)
  • Naïve datetime (UTC) → struct_time (UTC, s):
    dt.utctimetuple()
  • Aware datetime → struct_time (UTC, s):
    dt.utctimetuple()
  • POSIX → Naïve datetime (local):
    datetime.fromtimestamp(t, None)
    (may fail in certain conditions, see comment from jfs below)
  • struct_time (UTC) → Naïve datetime (local, s):
    datetime.datetime(struct_time[:6], tzinfo=UTC).astimezone(tz).replace(tzinfo=None)
    (can't represent leap seconds, see comment from jfs)
  • Naïve datetime (UTC) → Naïve datetime (local):
    dt.replace(tzinfo=UTC).astimezone(tz).replace(tzinfo=None)
  • Aware datetime → Naïve datetime (local):
    dt.astimezone(tz).replace(tzinfo=None)
  • POSIX → Naïve datetime (UTC):
    datetime.utcfromtimestamp(t)
  • struct_time (UTC) → Naïve datetime (UTC, s):
    datetime.datetime(*struct_time[:6])
    (can't represent leap seconds, see comment from jfs)
  • Naïve datetime (local) → Naïve datetime (UTC):
    stz.localize(dt, is_dst=None).astimezone(UTC).replace(tzinfo=None)
    (exception during DST transitions, see comment from jfs)
  • Aware datetime → Naïve datetime (UTC):
    dt.astimezone(UTC).replace(tzinfo=None)
  • POSIX → Aware datetime:
    datetime.fromtimestamp(t, tz)
    (may fail for non-pytz timezones)
  • struct_time (UTC) → Aware datetime (s):
    datetime.datetime(struct_time[:6], tzinfo=UTC).astimezone(tz)
    (can't represent leap seconds, see comment from jfs)
  • Naïve datetime (local) → Aware datetime:
    stz.localize(dt, is_dst=None)
    (exception during DST transitions, see comment from jfs)
  • Naïve datetime (UTC) → Aware datetime:
    dt.replace(tzinfo=UTC)

Source: taaviburns.ca

def local_to_utc(t):
    secs = time.mktime(t)
    return time.gmtime(secs)

def utc_to_local(t):
    secs = calendar.timegm(t)
    return time.localtime(secs)

Source: http://feihonghsu.blogspot.com/2008/02/converting-from-local-time-to-utc.html

Example usage from bd808: If your source is a datetime.datetime object t, call as:

local_to_utc(t.timetuple())

I'm having good luck with dateutil (which is widely recommended on SO for other related questions):

from datetime import *
from dateutil import *
from dateutil.tz import *

# METHOD 1: Hardcode zones:
utc_zone = tz.gettz('UTC')
local_zone = tz.gettz('America/Chicago')
# METHOD 2: Auto-detect zones:
utc_zone = tz.tzutc()
local_zone = tz.tzlocal()

# Convert time string to datetime
local_time = datetime.strptime("2008-09-17 14:02:00", '%Y-%m-%d %H:%M:%S')

# Tell the datetime object that it's in local time zone since 
# datetime objects are 'naive' by default
local_time = local_time.replace(tzinfo=local_zone)
# Convert time to UTC
utc_time = local_time.astimezone(utc_zone)
# Generate UTC time string
utc_string = utc_time.strftime('%Y-%m-%d %H:%M:%S')

(Code was derived from this answer to Convert UTC datetime string to local datetime)

One more example with pytz, but includes localize(), which saved my day.

import pytz, datetime
utc = pytz.utc
fmt = '%Y-%m-%d %H:%M:%S'
amsterdam = pytz.timezone('Europe/Amsterdam')

dt = datetime.datetime.strptime("2012-04-06 10:00:00", fmt)
am_dt = amsterdam.localize(dt)
print am_dt.astimezone(utc).strftime(fmt)
'2012-04-06 08:00:00'

I've had the most success with python-dateutil:

from dateutil import tz

def datetime_to_utc(date):
    """Returns date in UTC w/o tzinfo"""
    return date.astimezone(tz.gettz('UTC')).replace(tzinfo=None) if date.tzinfo else date

import time

import datetime

def Local2UTC(LocalTime):

    EpochSecond = time.mktime(LocalTime.timetuple())
    utcTime = datetime.datetime.utcfromtimestamp(EpochSecond)

    return utcTime

>>> LocalTime = datetime.datetime.now()

>>> UTCTime = Local2UTC(LocalTime)

>>> LocalTime.ctime()

'Thu Feb  3 22:33:46 2011'

>>> UTCTime.ctime()

'Fri Feb  4 05:33:46 2011'

if you prefer datetime.datetime:

dt = datetime.strptime("2008-09-17 14:04:00","%Y-%m-%d %H:%M:%S")
utc_struct_time = time.gmtime(time.mktime(dt.timetuple()))
utc_dt = datetime.fromtimestamp(time.mktime(utc_struct_time))
print dt.strftime("%Y-%m-%d %H:%M:%S")

You can do it with:

>>> from time import strftime, gmtime, localtime
>>> strftime('%H:%M:%S', gmtime()) #UTC time
>>> strftime('%H:%M:%S', localtime()) # localtime

How about - 

time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime(seconds))

if seconds is None then it converts the local time to UTC time else converts the passed in time to UTC.

Simple

I did it like this:

>>> utc_delta = datetime.utcnow()-datetime.now()
>>> utc_time = datetime(2008, 9, 17, 14, 2, 0) + utc_delta
>>> print(utc_time)
2008-09-17 19:01:59.999996

Fancy Implementation

If you want to get fancy, you can turn this into a functor:

class to_utc():
    utc_delta = datetime.utcnow() - datetime.now()

    def __call__(cls, t):
        return t + cls.utc_delta

Result: 

>>> utc_converter = to_utc()
>>> print(utc_converter(datetime(2008, 9, 17, 14, 2, 0)))
2008-09-17 19:01:59.999996

For getting around day-light saving, etc.

None of the above answers particularly helped me. The code below works for GMT.

def get_utc_from_local(date_time, local_tz=None):
    assert date_time.__class__.__name__ == 'datetime'
    if local_tz is None:
        local_tz = pytz.timezone(settings.TIME_ZONE) # Django eg, "Europe/London"
    local_time = local_tz.normalize(local_tz.localize(date_time))
    return local_time.astimezone(pytz.utc)

import pytz
from datetime import datetime

summer_11_am = datetime(2011, 7, 1, 11)
get_utc_from_local(summer_11_am)
>>>datetime.datetime(2011, 7, 1, 10, 0, tzinfo=<UTC>)

winter_11_am = datetime(2011, 11, 11, 11)
get_utc_from_local(winter_11_am)
>>>datetime.datetime(2011, 11, 11, 11, 0, tzinfo=<UTC>)

Using http://crsmithdev.com/arrow/

arrowObj = arrow.Arrow.strptime('2017-02-20 10:00:00', '%Y-%m-%d %H:%M:%S' , 'US/Eastern')

arrowObj.to('UTC') or arrowObj.to('local')

This library makes life easy :)

In python3:

pip install python-dateutil

from dateutil.parser import tz

mydt.astimezone(tz.gettz('UTC')).replace(tzinfo=None)

I found the best answer on another question here. It only uses python built-in libraries and does not require you to input your local timezone (a requirement in my case) 

import time
import calendar

local_time = time.strptime("2018-12-13T09:32:00.000", "%Y-%m-%dT%H:%M:%S.%f")
local_seconds = time.mktime(local_time)
utc_time = time.gmtime(local_seconds)

I'm reposting the answer here since this question pops up in google instead of the linked question depending on the search keywords.

in this case ... pytz is best lib

import pytz
utc = pytz.utc
yourdate = datetime.datetime.now()
yourdateutc = yourdate.astimezone(utc).replace(tzinfo=None)
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