setTimeZoneOffset is still a weak solution. A person must be able to set a timezone, like America/Los_Angeles and let the class take care of the actual offset, DST etc. KDateTime uses KTzfileTimeZone and related classes to directly select a timezone like this. For more information, see:

http://api.kde.org/4.0-api/kdelibs-apidocs/kdecore/html/group__timezones.html
and related pages.

You do not need to provide timezone info yourself. You can use packages like tzdata which can be preinstalled or you can (if feasible) package it with Qt like you do libpng, sqlite etc. tzdata is public domain.