How to convert from timestamp to date in Qt?

QDateTime.setTime_t

int unixTime = 1234567890;
QDateTime timestamp;
timestamp.setTime_t(unixTime);
qDebug() << timestamp.toString(Qt::SystemLocaleShortDate);

That should get you going. Like Matthew said, see QDateTime.setTime_t, as well as QDateTime.toString. The toString has an enumeration with several different options, as well as an overload where you can pass a string allowing however much customization you like.