Enter this into code block

<div id="timer"> 
    <div id="days"></div>
    <div id="hours"></div>
    <div id="minutes"></div>
    <div id="seconds"></div>
</div>

Enter this into custom CSS:

/* COUNTDOWN TIMER CSS */

#timer {
  margin: 0 auto;
  display: table;
  div {
    display: table-cell;
// STYLE NUMBERS
    font-size: 50px;
    letter-spacing: 2px;
    font-weight: 500;
    color: #f65959;
  padding: 20px;
    border-radius: 300px;
    text-align: center;
    span {
// STYLES DAYS, HOURS, MINUTES, SECONDS
      letter-spacing: 0px;
      color: #111111;
  font-size: 20px;
      margin-left: -4px;
      margin-top: 16px;
      display: block;
    }
  }
}
@media (max-width : 667px) {
  #timer {
    div {
      padding: 16px;
      font-size: 26px !important;
      span {
        margin-top: 2px;
        font-size: 12px !important;
      }
    }
  }
}

/* END OF COUNTDOWN TIMER CSS */

Enter this into Code Injector in page Header

<script src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script>
function makeTimer() {

var endTime = new Date("December 25, 2019 12:00:00 EST");
var endTime = (Date.parse(endTime)) / 1000;

var now = new Date();
var now = (Date.parse(now) / 1000);

var timeLeft = endTime - now;

var days = Math.floor(timeLeft / 86400);
var hours = Math.floor((timeLeft - (days * 86400)) / 3600);
var minutes = Math.floor((timeLeft - (days * 86400) - (hours * 3600 )) / 60);
var seconds = Math.floor((timeLeft - (days * 86400) - (hours * 3600) - (minutes * 60)));

if (hours < "10") { hours = "0" + hours; }
if (minutes < "10") { minutes = "0" + minutes; }
if (seconds < "10") { seconds = "0" + seconds; }

$("#days").html(days + "<span>Days</span>");
$("#hours").html(hours + "<span>Hours</span>");
$("#minutes").html(minutes + "<span>Minutes</span>");
$("#seconds").html(seconds + "<span>Seconds</span>");

}

setInterval(function() { makeTimer(); }, 1000);
</script>