How can I rotate an object around its local Y axis rather than the global Y axis using actionscript-3?

Question

I have a flat object, eventually it will be a circle, but for now I just have a 2d square on the stage. I tip it back 75 degrees, and then rotate it around the y axis.

I have this code which has the object rotating, but it rotates around the global Y axis. But I want it to rotate around it's own Y axis which is tipped 15 degrees from vertical.

box_mc.rotationY=0;
box_mc.rotationX=-75;

addEventListener(Event.ENTER_FRAME, onYRotate);

function onYRotate(e:Event):void{
    box_mc.rotationY+=2;
}

Can someone help me change my code so it rotates on its own Y axis?