this one
rate = -1;
clockStart = 300;
function padZero(n){
if (n < 10) return "0" + n else return "" + n
}
clockTime = Math.max(clockStart + rate*(time - inPoint),0);
t = Math.floor(clockTime);
min = Math.floor((t%3600)/60);
sec = Math.floor(t%60);
min + ":" + padZero(sec)