[CorC]

предлагаю так:

stop();
tm_h0 = 15;
tm_m0 = 30;
tm_s0 = 0;
setInterval(function () {
	var tm = new Date();
	var tm_s1 = tm.getSeconds();
	var tm_m1 = tm.getMinutes();
	var tm_h1 = tm.getHours();
	var i=tm_h0-tm_h1;
	var j=tm_m0-tm_m1
	var k=tm_s0-tm_s1;
	if(k<0){
		j--;
		k=60+k;
	}
	if(j<0){
		i--;
		j=60+j;
	}
	timer = i+":"+j+":"+k;
	trace(timer);
}, 1000);