I have a date that I build: from datetime import datetime from datetime import tzinfo test = '2013-03-27 23:05' test2 = datetime.strptime(test,'%Y-%m-%d %H:%M') >>> test2 datetime.datetime(2013, 3, 27, 23, 5) >>> test2.replace(tzinfo=EST) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'EST' is not defined >> test2.replace(tzinfo=UTC) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'UTC' is not defined I can't find documentation on the list of time zone names that I can assign to tzinfo in the replace.tzinfo= call. I have read through the following and there is nothing: http://docs.python.org/2/library/datetime.html#tzinfo-objects Edit: The solution provided by unutbu got the following: >>> test = '2013-03-27 00:05' >>> test '2013-03-27 00:05' >>> test2 = dt.datetime.strp(test, '%Y-%m-%d %H:%M') >>> test2 datetime.datetime(2013, 3, 27, 0, 5) >>> est = pytz.timezone('US/Eastern') >>> utc = pytz.utc >>> print(est.localize(test2)) 2013-03-27 00:05:00-04:00 >>> print(utc.localize(test2)) 2013-03-27 00:05:00+00:00 >>> print(est.localize(test2,is_dst=False)) 2013-03-27 00:05:00-04:00 >>> print(est.localize(test2,is_dst=True)) 2013-03-27 00:05:00-04:00 >>> As you can see even when I provide the is_dst= flag the offset is still '-04:00', which is EDT and not EST. The documentation shows the following: If you insist on working with local times, this library provides a facility for constructing them unambiguously: http://pytz.sourceforge.net/#problems-with-localtime >>> loc_dt = datetime(2002, 10, 27, 1, 30, 00) >>> est_dt = eastern.localize(loc_dt, is_dst=True) >>> edt_dt = eastern.localize(loc_dt, is_dst=False) >>> print(est_dt.strftime(fmt) + ' / ' + edt_dt.strftime(fmt)) 2002-10-27 01:30:00 EDT-0400 / 2002-10-27 01:30:00 EST-0500 eastern was defined earlier in the documentation as eastern = timezone('US/Eastern') This seems to indicate that the is_dst= flag should further specify whether day light savings is specified or not. I would appreciate help on why this isn't working in my case. Continue reading...