JSFiddle - React, Tailwind, and code Playground

HTML

<div id="timeleft"></div>

JavaScript

var timeStr, dateStr, ostStr, x, tt;
all_secc=300+1;
function clock() {
    all_secc = all_secc - 1;
    minn = Math.floor(all_secc / 60);
    secc = all_secc - Math.floor(minn * 60);
    if (secc < 10) {
        secc = "0" + secc;
    }
    if (all_secc > 0) {
        document.getElementById("timeleft").innerHTML = minn + (":" + secc);
    } else {
        location.href = "test_rez.php";
    }
    Timer = setTimeout(clock, 1000);
}
clock();