d = thisComp.duration;
min = Math.floor(d / 60);
sec = Math.floor(d % 60);
fr = Math.floor((d % 1) / thisComp.frameDuration);
(min < 10 ? "0" : "") + min + ":" + (sec < 10 ? "0" : "") + sec + ":" + (fr < 10 ? "0" : "") + fr;