if you do all calculations with ``relativedelta`` in UTC, then sub-daily transitions across DST fail, e.g.


    >> t = Timestamp("2013-11-02 22:00", tz='US/Pacific')
    Timestamp('2013-11-02 22:00:00-0700', tz='US/Pacific')
    # fall back adds an extra hour. so 10pm + 8hrs should be 5am
    >> Timestamp("2013-11-02 22:00", tz='US/Pacific') + DateOffset(hours=8)
    Timestamp('2013-11-03 06:00:00-0800', tz='US/Pacific')