cesium polygon+callbackproperty

var viewer = new Cesium.Viewer('cesiumContainer');

function callback(time, result) {
    
    var txf = Math.floor(time.secondsOfDay);
    if (txf %2 === 1) {
        
    result = Cesium.Cartesian3.fromDegreesArray([-115.0, 37.0,
                                                        -115.0, 32.0,
                                                        -107.0, 33.0,
                                                        -102.0, 31.0,
                                                        -102.0, 35.0]);
    } else {
    result = Cesium.Cartesian3.fromDegreesArray([-116, 37.0,
                                                        -115.0, 32.0,
                                                        -107.0, 33.0,
                                                        -102.0, 31.0,
                                                        -102.0, 35.0]);
    }
    
    return result;
}

var hp = new Cesium.CallbackProperty(callback, false);

var redPolygon = viewer.entities.add({
    name : 'Red polygon on surface',
    polygon : {
        hierarchy : hp,
        material : Cesium.Color.RED
    }
});


viewer.zoomTo(viewer.entities);