[VERDANT]

<button onclick = "startTimer()">Старт</button>
<span id="mT">00:00:10</span>
<script>
function startTimer () {
t = setInterval (function() {
var s = document.getElementById ('mT'),
f = function (x) {
return (x / 100).toFixed (2).substr (2)
},
d = ':',
y = s.innerHTML.split (d),
z = --y [2] + y [0] * 3.6e3 + y [1] * 60;

if (!z) clearInterval (t);
s.innerHTML = [
f (Math.floor (z / 3600)),
f (Math.floor (z % 3600 / 60)),
f (z % 3600 % 60)
].join (d);
}, 1000);
<script>