<input id="date" type="date">
<input id="time" type="time">
<button id="start">Start</button>
<div id="elapsed"></div>

document.getElementById("start").addEventListener("click", started);
var elapsed = document.getElementById("elapsed");

function started() {
  if (!document.getElementById("date").value) {elapsed.textContent = "Enter date"; return;}
  if (!document.getElementById("time").value) {elapsed.textContent = "Enter time"; return;}
  
  let now = new Date();
  let set = new Date(document.getElementById("date").value + "T" +  document.getElementById("time").value);
  
  let sec = (set - now) / 1000;
  let d = parseInt(sec / 86400);
  let h = parseInt(sec % 86400 / 3600);
  let m = parseInt(sec % 86400 % 3600 / 60);
  let s = parseInt(sec % 86400 % 3600 % 60);
  elapsed.textContent = d + " days " + h + " hours " + m + " minutes " + s + " seconds";
  setTimeout(started, 1000);
}