countdown timer

countdown_timer.html

<div id="timer_stock">
  <span class="date_finish" style="display: none;" >2016-12-31</span>
  <span class="days">00</span>
  <span class="hours">00</span>
  <span class="minutes">00</span>
  <span class="seconds">00</span>
</div>

countdown_timer.js

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 ) {
  var clock = document.getElementById( id );
  var endtime = clock.querySelector( '.date_finish' ).innerHTML;
  var daysSpan = clock.querySelector( '.days' );
  var hoursSpan = clock.querySelector( '.hours' );
  var minutesSpan = clock.querySelector( '.minutes' );
  var secondsSpan = clock.querySelector( '.seconds' );
  function updateClock() {
    var t = getTimeRemaining( endtime );
    daysSpan.innerHTML = ( '0' + t.days ).slice( -2 );
    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 );
}

initializeClock( 'timer_stock' );