We use ICU calendar to calculate timezone offset, and ICU calendar use 0..11 to present the months, however the datetime we passed is QDateTime, which use 1-12 to present months, that's why I mont()-1 here.
FYI:
[calendar.h Line 219)[http://www.icu-project.org/apiref/icu4c/calendar_8h_source.html)
[QDate::month()](http://qt-project.org/doc/qt-4.8/qdate.html#month)