Fix opposite sign of timezone

time.timezone returns offset of the opposite
sign from what we normally use in UTC offset

For example,
CST is actually UTC +0800
http://www.timeanddate.com/library/abbreviations/timezones/asia/cst.html
```python
>>> import time
>>> time.tzname
('CST', 'CST')
>>> "%+05d" % (time.timezone / 36)
-0800
```