Convert a local datetime object to a UTC string represestation, taking daylight savings time in to account

localtime_with_timezone_to_utc_datestring.py

"""
Convert a local datetime object to a UTC string represestation, taking 
daylight savings time in to account. Here's an example - the clocks change 
at 2am on March 13th 2011.

>>> to_utc_datestring(datetime.datetime(2011, 3, 11, 17, 30))
'20110311T233000Z'
>>> to_utc_datestring(datetime.datetime(2011, 3, 15, 17, 30))
'20110315T223000Z'

You can use http://www.timezoneconverter.com/cgi-bin/tzc.tzc to check results
"""
import pytz # http://pytz.sourceforge.net/

def to_utc_datestring(localtime, timezone='America/Chicago'):
    tz = pytz.timezone(timezone)
    return (localtime.replace(
        tzinfo = tz
    ).astimezone(
        pytz.utc
    ) - tz.dst(localtime)).strftime(
        '%Y%m%dT%H%M%SZ'
    )