countdown timer with reference to timestamp

HTML

<h1>
  This code compares 2 timestamps. Calculates the difference in minutes and seconds and starts a cauntdown timer based on the time difference.
</h1>
<div > Present time = <span id="nowTime"></span></div>
<div > Reference TimeStamp = <span id="thenTime"></span></div>
<div > Difference in time = <span id="alertDiv"></span></div>
<div > seconds = <span id="secs"></span></div>
<div > mins = <span id="mins"></span></div>
<div class="timer">Time left = <span id="timer"></span></div>

CSS

.timer{
  color:#00a84e;
  font-size: 40px;
  
}

JavaScript

var date1 = new Date();

var date3 = '1460540284'
var date2 = new Date("2015-12-09T02:34:51.199Z");
var timeDiff = Math.abs((date1.getTime()-Date.parse("2015-12-09T02:34:51.199Z"))/60000);
var duration = Math.abs((date1.getTime()-Date.parse("2015-12-09T02:34:51.199Z")));
seconds = parseInt((duration/1000)%60);
minutes = parseInt((duration/(60000))%60);
minutes = (minutes < 10) ? "0" + minutes : minutes;
seconds = (seconds < 10) ? "0" + seconds : seconds;
document.getElementById('secs').innerHTML =seconds;
document.getElementById('mins').innerHTML =minutes;
document.getElementById('nowTime').innerHTML =date1;
document.getElementById('thenTime').innerHTML =date2;
document.getElementById('alertDiv').innerHTML =timeDiff;


document.getElementById('timer').innerHTML =
  minutes + ":" + seconds;


startTimer();

function startTimer() {
  var presentTime = document.getElementById('timer').innerHTML;
  var timeArray = presentTime.split(/[:]+/);
  var m = timeArray[0];
  var s = checkSecond((timeArray[1] - 1));
  if(s==59){m=m-1}
  //if(m<0){alert('timer completed')}
  
  document.getElementById('timer').innerHTML =
    m + ":" + s;
  setTimeout(startTimer, 1000);
}

function checkSecond(sec) {
  if (sec < 10 && sec >= 0) {sec = "0" + sec}; // add zero in front of numbers < 10
  if (sec < 0) {sec = "59"};
  return sec;
}