Negative dates in C++ are always one year further into the future when converted to JavaScript.

QML from the attached project:

import QtQuick 2.0
import test 1.0

Item {
    id: root

    QuickDate {
        Component.onCompleted: {
            console.log("  actual date = " + date);
            console.log("expected date = " + new Date(-2000, 0, 1));
        }
    }
}

Output:

actual date = Mon Jan 01 -1999 00:00:00 GMT+0100 (CET)
expected date = Sat Jan 01 -2000 00:00:00 GMT+0100 (CET)