After Effects Countdown timer script (by: tutvid)

timer.js

rateofSpeed = 1;
clockStartTime = thisComp.layer("Timer Duration").effect("Duration")("Slider")*60;

function addZero(n) {
	if(n<10) { return "0" + n } else { return n; }
}

function noZero(n) {
	if(n > 0) { return n+":" } else { return "";}
}

clockTimeNumber = Math.floor(clockStartTime - rateofSpeed*time);

minutes = noZero(Math.floor(clockTimeNumber/60));
seconds = clockTimeNumber%60;
minutes+addZero(seconds);