Simple Timer

My basic game is a series of click your way through questions. So the timer should start at frame 72, and end at frame 150.

start_time = getTimer();
countdown = 7200000;
onEnterFrame = function () {
elapsed_time = getTimer()-start_time;
root.count.text = timeto_string(elapsed_time);
root.countdown.text = time_to_string(root.countdown-elapsedtime);
};
function time_to_string(time_to_convert) {
elapsed_hours = Math.floor(time_to_convert/3600000);
remaining = time_to_convert-(elapsed_hours*3600000);
elapsed_minutes = Math.floor(remaining/60000);
remaining = remaining-(elapsed_minutes*60000);
elapsed_seconds = Math.floor(remaining/1000);
remaining = remaining-(elapsed_seconds*1000);
elapsed_fs = Math.floor(remaining/10);
if (elapsed_hours<10) {
hours = "0"+elapsed_hours.toString();
} else {
hours = elapsed_hours.toString();
}
if (elapsed_minutes<10) {
minutes = "0"+elapsed_minutes.toString();
} else {
minutes = elapsed_minutes.toString();
}
if (elapsed_seconds<10) {
seconds = "0"+elapsed_seconds.toString();
} else {
seconds = elapsed_seconds.toString();
}
if (elapsed_fs<10) {
hundredths = "0"+elapsed_fs.toString();
} else {
hundredths = elapsed_fs.toString();
}
return hours+":“minutes”:“seconds”:"+hundredths;
}

It starts, but i dont know how i can get it to stop on a certain frame?

Clear the onEnterFrame function on frame 150 using a timeline script. onEnterFrame=null maybe, or onEnterFrame=function{}.

Or poll for the currentFrame property

function onEnterFrame=function {

  if(!currentFrame>=150)

 {

    //do somecode.

}

//else it does nothing

}