>>> 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)'
        Nr9