How to convert local time string to UTC?


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.

1
273
3/9/2019 11:07:30 AM

Accepted Answer

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

61
11/1/2011 7:07:38 PM

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")

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