JS Date.getTimezoneOffset() always returns zero when using TZ environment variable

With the following minimal example:

import QtQuick 2.12
Item {
    Component.onCompleted: console.log("TimezoneOffset:", (new Date()).getTimezoneOffset())
}

This shows a correct timezone offset for my system settings:

$ ~/Qt/5.12.4/gcc_64/bin/qmlscene TimezoneOffsetTest.qml
qml: TimezoneOffset: 240

But when run with the timezone set via the TZ environment variable, zero is always returned:

$ TZ=UTC+10 ~/Qt/5.12.4/gcc_64/bin/qmlscene TimezoneOffsetTest.qml
qml: TimezoneOffset: 0