gistfile1.txt

In [1]: import pytz

In [2]: from datetime import datetime, date, time, timedelta

In [3]: now = datetime.utcnow().replace(tzinfo=pytz.UTC)

In [4]: now
Out[4]: datetime.datetime(2015, 4, 3, 18, 17, 45, 509236, tzinfo=<UTC>)

In [5]: localnow = now.astimezone(pytz.timezone('US/Eastern'))

In [6]: localnow
Out[6]: datetime.datetime(2015, 4, 3, 14, 17, 45, 509236, tzinfo=<DstTzInfo 'US/Eastern' EDT-1 day, 20:00:00 DST>)

In [7]: normalizednow = pytz.timezone('US/Eastern').normalize(localnow)

In [8]: normalizednow
Out[8]: datetime.datetime(2015, 4, 3, 14, 17, 45, 509236, tzinfo=<DstTzInfo 'US/Eastern' EDT-1 day, 20:00:00 DST>)

In [9]: today_midnight = datetime.combine(date.today(), time(0, tzinfo=pytz.timezone('US/Eastern')))

In [10]: today_midnight_in_utc = today_midnight.astimezone(pytz.UTC)

In [11]: today_midnight_in_utc
Out[11]: datetime.datetime(2015, 4, 3, 5, 0, tzinfo=<UTC>)

In [12]: today_midnight
Out[12]: datetime.datetime(2015, 4, 3, 0, 0, tzinfo=<DstTzInfo 'US/Eastern' EST-1 day, 19:00:00 STD>)

In [13]: today_midnight.dst()
Out[13]: datetime.timedelta(0)

In [14]: localnow.dst()
Out[14]: datetime.timedelta(0, 3600)

In [15]: better_today_midnight = pytz.timezone('US/Eastern').localize(datetime.combine(date.today(), time()))

In [16]: better_today_midnight_utc = better_today_midnight.astimezone(pytz.UTC)

In [17]: better_today_midnight
Out[17]: datetime.datetime(2015, 4, 3, 0, 0, tzinfo=<DstTzInfo 'US/Eastern' EDT-1 day, 20:00:00 DST>)

In [18]: better_today_midnight_utc
Out[18]: datetime.datetime(2015, 4, 3, 4, 0, tzinfo=<UTC>)

In [19]: spring_forward = datetime(2015, 3, 8, 2, 0, 0)

In [20]: fall_back = datetime(2015, 11, 1, 2, 0, 0)

In [21]: pytz.timezone('US/Eastern').localize(spring_forward + timedelta(seconds=60), is_dst=None)
---------------------------------------------------------------------------
NonExistentTimeError                      Traceback (most recent call last)
<ipython-input-21-b255774bc7b0> in <module>()
----> 1 pytz.timezone('US/Eastern').localize(spring_forward + timedelta(seconds=60), is_dst=None)

/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pytz/tzinfo.pyc in localize(self, dt, is_dst)
    325             # If we refuse to guess, raise an exception.
    326             if is_dst is None:
--> 327                 raise NonExistentTimeError(dt)
    328 

In [22]: pytz.timezone('US/Eastern').localize(fall_back - timedelta(seconds=60), is_dst=None)
---------------------------------------------------------------------------
AmbiguousTimeError                        Traceback (most recent call last)
<ipython-input-22-92196f229dd3> in <module>()
----> 1 pytz.timezone('US/Eastern').localize(fall_back - timedelta(seconds=60), is_dst=None)

/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pytz/tzinfo.pyc in localize(self, dt, is_dst)
    347         # ambiguous case
    348         if is_dst is None:
--> 349             raise AmbiguousTimeError(dt)
    350 
    351         # Filter out the possiblilities that don't match the requested

AmbiguousTimeError: 2015-11-01 01:59:00