'Correct the timezone information on the given datetime
If date arithmetic crosses DST boundaries, the tzinfo
is not magically adjusted. This method normalizes the
tzinfo to the correct one.
To test, first we need to do some setup
>>> from pytz import timezone
>>> utc = timezone(\'UTC\')
>>> eastern = timezone(\'US/Eastern\')
>>> fmt = \'%Y-%m-%d %H:%M:%S %Z (%z)\'
We next create a datetime right on an end-of-DST transition point,
the instant when the wallclocks are wound back one hour.
>>> utc_dt = datetime(2002, 10, 27, 6, 0, 0, tzinfo=utc)
>>> loc_dt = utc_dt.astimezone(eastern)
>>> loc_dt.strftime(fmt)
\'2002-10-27 01:00:00 EST (-0500)\'
Now, if we subtract a few minutes from it, note that the timezone
information has not changed.
>>> before = loc_dt - timedelta(minutes=10)
>>> before.strftime(fmt)
\'2002-10-27 00:50:00 EST (-0500)\'
But we can fix that by calling the normalize method
>>> before = eastern.normalize(before)
>>> before.strftime(fmt)
\'2002-10-27 01:50:00 EDT (-0400)\'
The supported method of converting between timezones is to use
datetime.astimezone(). Currently, normalize() also works:
>>> th = timezone(\'Asia/Bangkok\')
>>> am = timezone(\'Europe/Amsterdam\')
>>> dt = th.localize(datetime(2011, 5, 7, 1, 2, 3))
>>> fmt = \'%Y-%m-%d %H:%M:%S %Z (%z)\'
>>> am.normalize(dt).strftime(fmt)
\'2011-05-06 20:02:03 CEST (+0200)\''

def normalize(self, dt):

    if (dt.tzinfo is None):
        raise ValueError('Naive    time    -    no    tzinfo    set')
    offset = dt.tzinfo._utcoffset
    dt = dt.replace(tzinfo=None)
    dt = (dt - offset)
    return self.fromutc(dt)
