timeSpanStart?

I'm trying to write a function that saves a thumbnail image from my comp. I'm getting tripped up with errors when trying to set the start of the time span and the duration. Is the timeSpanStart the same as comp.displayStartTime in seconds? Or is timeSpanStart "0"? I seem to be getting conflicting values. When I query it, I get 0. But when I try to add in a value it seems to add it to displayStartTime. Very confused.

function renderMidpointStillFrame(comp) {
    var compDuration = comp.duration;
    var midpointFrame = compDuration / 2;
    $.writeln("Calculating midpoint of " + midpointFrame + " from " + compDuration);
    app.project.renderQueue.items.add(comp);
    app.project.renderQueue.item(1).timeSpanDuration = comp.frameDuration;
    $.writeln("Time Span start of RQ is " + app.project.renderQueue.item(1).timeSpanStart);
    app.project.renderQueue.item(1).timeSpanStart =+ midpointFrame;
    $.writeln("Time Span start of RQ is now " + app.project.renderQueue.item(1).timeSpanStart);


}