JSFiddle - React, Tailwind, and code Playground

by Greg Bowler

HTML

<p>Original date: <span id="original"></span></p>
<p>Modified date: <span id="modified"></span></p>

JavaScript

var dt = new Date(2012, 2, 1); // March 1st 2012.

document.getElementById("original").textContent = dt.toString();

var milliseconds = dt.getTime();

// Calculate the number of milliseconds in one day:
var oneDay = 1000 * 60 // 1 minute
                  * 60 // 1 hour
                  * 24;// 1 day

// Set the millisecond data of your Date by subtracting oneDay from the original.
dt.setTime(milliseconds -= oneDay);

document.getElementById("modified").textContent = dt.toString();