Timer

In order to do that you will need to set a variable for your timer and then use a conditional to stop your clock.

For example: 

I suppose you are using an interval to move your hands.

var interval = setInterval(runtheTimer, 1000);   // every second the second hand moves)

(runTheTimer is the name of the function you use to run your clock)

So after 45 seconds you will clear the interval

var timeCounted = 0;  // initialize the variable outside of the function

add this to the function that runs your clock.

everySec += 1000;

if(everySec == 45000){
clearInterval(interval)
}

You could also generalize this by making anoter function an passing an argument that will be the amount of seconds you want you clock to stop.

var everySec = 0;

function stopTheClock(howLong) {
	everySec += 1000;
	if (everySec == howLong) {
	     clearInterval(interval)
	}
}

them in your runtheClock function you can add the function with the time you want

// for 30 seconds

stopTheClock(30000);

// for 45 seconds

stopTheClock(45000);

etc...

Hope this helps depending on how you run your clock.