auto form submition at countdown end javascript

/*

HTML needed in the body before Javascript
<div id="demo"></div>
<form method="post" id="username">
<input type="text" name="usename" placeholder="Enter Username">
</form>

<?php

if ($_SERVER['REQUEST_METHOD'] == 'POST) {
echo $_POST['username'];
}

?>

*/

function timeRemaining (id, stoptime) {
      // Set the date we're counting down to
      var countDownDate = new Date(stoptime).getTime();

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

        // Get today's date and time
        var now = new Date().getTime();

        // Find the distance between now and the count down date
        var timediffer = countDownDate - now;

        // 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(id).innerHTML = days + "d " + hours + "h "
            + minutes + "m " + seconds + "s ";

        // If the count down is over, write some text
        if (timediffer < 0) {
          clearInterval(x);
          $('#username').submit();
          document.getElementById(id).innerHTML = "Time-up Form is submiting";
        }

      }, 1000);
    }
    timeRemaining('demo', 'Dec 2, 2020 14:21:00');