function ShowTime() {
  var now = new Date();
  var hrs = 18-now.getHours();
  var mins = 60-now.getMinutes();
  var secs = 60-now.getSeconds();
      timeLeft = "" +hrs+' hours '+mins+' minutes '+secs+' seconds';
  $("#countdown").html(timeLeft);
}

var countdown;
function StopTime() {
    clearInterval(countdown);
    
}

setInterval(ShowTime ,1000);

Hello, World!