Took me like 15 minutes to figure this out. It's your keyboard. Switch it to English. The quotes are not of the correct type. Also, your minus from the third line of code its not a minus. Here is the working code:

rateOfSpeed=1;

clockStartTime =thisComp.layer("slider").effect("Slider Control")("Slider")*60;

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

function addZero(n) {

if (n<10) return "0" + n else return n;

}

minutes = Math.floor(clockTimeNumber/60);

seconds = clockTimeNumber%60;

if (clockStartTime > 0 && time < clockStartTime) {

addZero(minutes) + ":" + addZero(seconds);

} else {"00:00"}