JSFiddle - React, Tailwind, and code Playground

HTML

<p id="output"></p>

JavaScript

function calcTime(offset) {
    var d = new Date();
    var utc = d.getTime() + (d.getTimezoneOffset() * 60000);
    var nd = new Date(utc + (1000*offset));

    document.getElementById("output").innerHTML =
        "The local time is " + nd.toLocaleString();
}

calcTime(-28800.0);