datetime -> DST offset in minutes east of UTC.

def dst(self, dt):
        """datetime -> DST offset in minutes east of UTC."""
        tt = _localtime(_mktime((dt.year, dt.month, dt.day,
                 dt.hour, dt.minute, dt.second, dt.weekday(), 0, -1)))
        if tt.tm_isdst > 0: return _dstdiff
        return _zero