how to set a time on a timer using javascript code example

Example 1: How to make a timer in javascript

// this example takes 2 seconds to run
const start = Date.now();

// After a certain amount of time, run this to see how much time passed.
const milliseconds = Date.now() - start;

console.log('Seconds passed = ' + millis / 1000);
// Seconds passed = *Time passed*

Example 2: javascript countdown timer including days

var countDownDate = new Date("Jul 25, 2021 16:37:52").getTime();

var myfunc = setInterval(function() {
    var now = new Date().getTime();
    var timeleft = countDownDate - now;
        
    var days = Math.floor(timeleft / (1000 * 60 * 60 * 24));
    var hours = Math.floor((timeleft % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
    var minutes = Math.floor((timeleft % (1000 * 60 * 60)) / (1000 * 60));
    var seconds = Math.floor((timeleft % (1000 * 60)) / 1000);
    
    }, 1000)