I create a QDateTime :
QDateTime(QDate(1970,01,01), QTime(0,0,0,0))
and when we are in summer hour I would create
QDateTime(QDate(1970,01,01), QTime(01,0,0,0))
How know if we are in summer hour or winter hour ?
A quick Google search and you can read all about the answer here: - http://www.zetcode.com/gui/qt4/datetime/
Just search for 'daylight saving' in the article.