JSFiddle - React, Tailwind, and code Playground

HTML

<div id="timer1"></div>
<div id="timer2"></div>

JavaScript

// update all timer
function updateTimer() {
    for (var i in aTimer) {
        var oTimer = document.getElementById(aTimer[i].sId);
        var iSeconds = parseInt((aTimer[i].iFinished - Date.now()) / 1000);
        oTimer.innerHTML = iSeconds;
    }
}

// Init all timers with DOM-id and finish time
var aTimer = [
    { sId: 'timer1', iFinished: Date.now() + 46800000 },
    { sId: 'timer2', iFinished: Date.now() + 780000}
];

// call display update
setInterval(function() {
    updateTimer();
}, 333);