Create a countdown timer in javascript that starts countdown after 1 hr

I'm trying to create a page where a 1 hr video displays. Towards the end of the video, lets say at 53 min, I would like a countdown timer to display with a link.

I've searched the internet to try to find a solution, but I have only found a reference that starts a countdown timer immediately, as seen here.

NOTE: I would also like to take the current time (once the timer starts) so I can pass it in php as a $_GET variable.

This is my HTML:

        <h1>Countdown Clock</h1>
<div id="clockdiv">
  <!-- <div>
    <span class="days"></span>
    <div class="smalltext">Days</div>
  </div> -->
  <div>
    <span class="hours"></span>
    <div class="smalltext">Hours</div>
  </div>
  <div>
    <span class="minutes"></span>
    <div class="smalltext">Minutes</div>
  </div>
  <div>
    <span class="seconds"></span>
    <div class="smalltext">Seconds</div>
  </div>
</div>

This is my JavaScript:

            function getTimeRemaining(endtime) {
      var t = Date.parse(endtime) - Date.parse(new Date());
      var seconds = Math.floor((t / 1000) % 60);
      var minutes = Math.floor((t / 1000 / 60) % 60);
      var hours = Math.floor((t / (1000 * 60 * 60)) % 24);
      var days = Math.floor(t / (1000 * 60 * 60 * 24));
      return {
        'total': t,
        'days': days,
        'hours': hours,
        'minutes': minutes,
        'seconds': seconds
      };
    }

    function initializeClock(id, endtime) {
      var clock = document.getElementById(id);
      // var daysSpan = clock.querySelector('.days');
      var hoursSpan = clock.querySelector('.hours');
      var minutesSpan = clock.querySelector('.minutes');
      var secondsSpan = clock.querySele`enter code here`ctor('.seconds');

      function updateClock() {
        var t = getTimeRemaining(endtime);

        // daysSpan.innerHTML = t.days;
        hoursSpan.innerHTML = ('0' + t.hours).slice(-2);
        minutesSpan.innerHTML = ('0' + t.minutes).slice(-2);
        secondsSpan.innerHTML = ('0' + t.seconds).slice(-2);

        if (t.total <= 0) {
          clearInterval(timeinterval);
        }
      }

      updateClock();
      var timeinterval = setInterval(updateClock, 1000);
      return
    }

    var currentdate = new Date(); 
    var dateTime =  (currentdate.getTime()/1000)/60;
    var startTimer = (dateTime)+.1;



    var deadline = new Date(Date.parse(new Date()) +60 * 60 * 1000);
    initializeClock('clockdiv', deadline);



    Any help would be appreciated. 
    Thanks!