Qml date behaves unexpectedly for negative years

import QtQuick 2.0
Item {
    width: 360
    height: 360

    property date date1: new Date()
    property date date2: new Date()

    Component.onCompleted: {
        console.log("date1 initial value: " + date1)
        var tmpDate = date1
        console.log("tmpDate initial value: " + tmpDate
        date1.setUTCFullYear(-1000)
        console.log("date1 -1000 year value: " + date1 + " (unexpected result!)")
        tmpDate.setUTCFullYear(-1000)
        console.log("tmpDate -1000 year value: " + tmpDate)
        date1 = tmpDate
        date2 = tmpDate
        console.log("date1 = tmpDate, date2 = tmpDate:")
        console.log("date1: " + date1 + " (unexpected year change, should be -1000!)")
        console.log("date2: " + date2 + " (unexpected year change, should be -1000!)")
        date2 = date1
        console.log("date2 = date1:")
        console.log("date1: " + date1)
        console.log("date2: " + date2 + " (notice the year increase!)")
        console.log("date1 == date2: " + (date1 == date2) + " (unexpected!)")
        date1 = date2
        console.log("date1 = date2:")
        console.log("date1: " + date1 + " (notice the year increase!)")
        console.log("date2: " + date2)
        console.log("date1 == date2: " + (date1 == date2) + " (unexpected!)")
    }
}