get timer start adding the started at time js

// Set the date we're counting down to
var start_at = new Date('2020-06-29 12:00:00');

// Update the count down every 1 second
var x = setInterval(function() {

    // Get todays date and time
    var now = new Date();
    
    // Find the distance between now an the count down date
    var distance = now - start_at;
	
    
    // Time calculations for days, hours, minutes and seconds
    var days = Math.floor(distance / (1000 * 60 * 60 * 24));
    var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
    var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
    var seconds = Math.floor((distance % (1000 * 60)) / 1000);
    
    // Output the result in an element with id="demo"
    document.getElementById("demo").value = hours + "h "
    + minutes + "m " + seconds + "s ";
}, 1000);