jsfl for setting playback range?

I have a cep that calls into js. I'd like to set the playback range to match to editorial's idea of the frame range for the shot. I need to write this function:

function setFrameRange(
    aStartFrame,
    anEndFrame) {

        // fl.getDocumentDOM().getTimeline().startFrame = aStartFrame;
        // fl.getDocumentDOM().getTimeline().duration = anEndFrame - aStartFrame
        return true;
}

but seems like all of the timeline variables that revolve around playback are read-only. What's the right way to do this?

Thanks in advance.

MO