HTML
<div id="timer">
<strong>-:-:-</strong>
</div>
JS
var timer_life = 90; //time in seconds (у тебя это будет 20 245 сек)
var timer = setInterval(function () {
timer_life -= 1;
if (timer_life == 0) {
clearInterval(timer);
//next actions, when timer was stopped
}
let w = Math.trunc(timer_life/3600),
m = Math.trunc((timer_life - (w*3600))/60),
s = Math.trunc(timer_life - ((w * 3600) + (m * 60)));
$('#timer > strong').text(w + ':' + m + ':' + s); //#timer > strong - is you selector, change it please
}, 1000)