Convert between timezones

[Akira Li]

Thomas 'PointedEars' Lahn <PointedEars at web.de> writes:

> [X-Post & F'up2 comp.unix.shell]
>
> Chris Angelico wrote:
>
>> On Fri, Jul 31, 2015 at 6:15 PM, Cameron Simpson <cs at zip.com.au> wrote:
>>> Actually, bash has no timezone support but the date command _does_, and
>>> probably neither better nor worse than Python. All one has to do is set
>>> the TZ environment variable, eg (untested):
>>>
>>>  _year_gmt=$( TZ=GMT date +%Y )
>> 
>> That's assuming that it's converting against the current system
>> timezone. I don't know how you'd use `date` to convert between two
>> arbitrary timezones. […]
>
> With POSIX date(1), ISTM all you could do is set the system time and for an 
> additional invocation the TZ variable accordingly for output.
>
> <http://pubs.opengroup.org/onlinepubs/9699919799/utilities/date.html>
>
> With GNU date(1):
>
> $ (tz_source="Asia/Dubai"; time_source="$(LC_TIME=C TZ=$tz_source date -d 
> "today 00:00 UTC+4" -Im)"; tz_target="America/Chicago"; echo "When it was 
> $time_source in $tz_source, it was $(LC_TIME=C TZ=$tz_target date -d 
> "$time_source") in $tz_target.")
> When it was 2015-07-31T00:00+0400 in Asia/Dubai, it was Thu Jul 30 15:00:00 
> CDT 2015 in America/Chicago.
>
> $ date --version
> date (GNU coreutils) 8.23
> […]
>

Here's a corresponding Python code. I haven't seen the beginning of the
discussion. I apologize if it has been already posted:

  #!/usr/bin/env python
  from datetime import datetime
  import pytz # $ pip install pytz
  
  source_tz, target_tz = map(pytz.timezone, ['Asia/Dubai', 'America/Chicago'])
  d = datetime.now(source_tz) # the current time in source_tz timezone
  midnight = source_tz.localize(datetime(d.year, d.month, d.day), is_dst=None)    
  fmt = "%Y-%m-%dT%H:%M:%S%z"
  print("When it was {:{fmt}} in {}, it was {:{fmt}} in {}".format(
      midnight, source_tz.zone, target_tz.normalize(midnight),
      target_tz.zone, fmt=fmt))

Output:

  When it was 2015-08-01T00:00:00+0400 in Asia/Dubai, it was
  2015-07-31T15:00:00-0500 in America/Chicago