Actionscript 3.0 :: Calculate Number Of Frames Between Two Labels?

I'm trying to create a function that looks at two Frame Labels and calculates the number of frames between them. This is what I've got so far:

Code: Select allfunction convertLabelsToFrames (targetClip:MovieClip, startFrameLabel:String, endFrameLabel:String):int
{
var time:int = targetClip.endFrameLabel.frame - targetClip.startFrameLabel.frame;
return time;
};

The problem I'm having is that you don't seem to be able to find a FrameLabel within a MovieClip in the simple sort of way that I've used above (I know the way I specify here won't work). It does seem, though, like there should be SOME way of finding a FrameLabel within a MovieClip.