JSFiddle - React, Tailwind, and code Playground

HTML

<label>Initial value:</label>
<span id="first"></span>
<br />
<label>New value:</label>
<span id="second"></span>

CSS

label {
  font-weight: bold;
}

JavaScript

Date.prototype.addSeconds = function(seconds) {
	this.setSeconds(this.getSeconds() + seconds);
};

var dt = new Date();
document.getElementById("first").innerHTML = dt;
dt.addSeconds(1234);
document.getElementById("second").innerHTML = dt;