Count Down Timer With User Input To Modify End Date

Is there a way to take this pre-existing code and make it so that instead of having the end date as a preset value. have it so that the user can select an end date using date inputs and time inputs. I there a way to do this by only Using HTML and Javascript with an onClick() function to avoid using PHP. Any help that I can get would be extremely helpful.

<p id="demo"></p>

<script>
// Set the date we're counting down to
var countDownDate = new Date("Jan 5, 2021 15:37:25").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 distance = 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("demo").innerHTML = days + "d " + hours + "h "
  + minutes + "m " + seconds + "s ";

  // If the count down is over, write some text 
  if (distance < 0) {
    clearInterval(x);
    document.getElementById("demo").innerHTML = "EXPIRED";
  }
}, 1000);
</script>

Solution

<!DOCTYPE HTML>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
p {
  text-align: center;
  font-size: 60px;
  margin-top: 0px;
}
</style>
</head>
<body>

<p id="demo"></p>
 End Date <input type="datetime-local" id="endDate">


<button onclick="countdownTimeStart()">Start Timer</button>

<script>
// Set the date we're counting down to
function countdownTimeStart(){
var endDate = document.getElementById('endDate').value;

var dateObject = new Date(endDate);
var countDownDate = new Date(dateObject).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 distance = 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("demo").innerHTML = days + "d " + hours + "h "
  + minutes + "m " + seconds + "s ";

  // If the count down is over, write some text 
  if (distance < 0) {
    clearInterval(x);
    document.getElementById("demo").innerHTML = "EXPIRED";
  }
}, 1000);
}
</script>

</body>
</html>