The problem

When formatting a QTime object in QML, the hour is wrong (due to DST inconsistencies):

// main.cpp
...
QQmlEngine *pQmlEngine = new QQmlEngine;
pQmlEngine->rootContext()->setContextProperty("myCESTDate", QTime(18,0));
...

// main.qml
...
console.log(myCESTDate); // prints "Mon Jun 8 17:00:00 1925 GMT+0200"
...

The reason

When converting a QTime object from C++ to qml daylight saving time (DST) is determined using localtime_r (call hierarchy: Heap::DateObject::init > UTC > DaylightSavingTA), which returns DST is true.

However, the qml (js) Date object is converted back to a QDateTime object using QDateTime::toTimeSpec, which internally calls epochMSecsToLocalTime, which does never sets DST for dates before 1970-01-01:

static bool epochMSecsToLocalTime(qint64 msecs, QDate *localDate, QTime *localTime,
                                  QDateTimePrivate::DaylightStatus *daylightStatus = 0)
{
    if (msecs < 0) {
        // Docs state any LocalTime before 1970-01-01 will *not* have any Daylight Time applied
        // Instead just use the standard offset from UTC to convert to UTC time
        qt_tzset();
        msecsToTime(msecs - qt_timezone() * 1000, localDate, localTime);
        if (daylightStatus)
            *daylightStatus = QDateTimePrivate::StandardTime;
        return true;
    } 
    ...
}

 On Windows, this does not seem to be a problem, but on macOS and iOS the above (wrong) behaviour is observed.

The solution

A solution may be to choose a date after 1970-01-01 (instead of 1925-6-8) when converting a QTime object to a qml (js) Date object (see function Heap::DateObject::init, including the implementation comment inside this function). 

Work around

A work around (without changing Qt's source code) would be to explicitly specify a (random or current) date after 1970-01-01, when passing a QTime object to qml:

...
pQmlEngine->rootContext()->setContextProperty("myCESTDate", QDateTime(QDate::currentDate(), QTime(18,0)));
...