Periodically Moving An Element in Silverlight

If i wanted to move an element, Y units of position, every T time interval...how would i do it???

Silverlight and Positioning Elements

Would like to be able to explain the difference between retrieving an elements position via:

GeneralTransform gt = lblRectPos.TransformToVisual(Application.Current.RootVisual);
result = gt.Transform(new Point(0, 0)).Y;

and

result =(double)_shape.GetValue(Canvas.TopProperty);