countdown.js

/**
 *
 * Simple Countdown Function
 * @param {num} duration - in Milliseconds
 * @param {string} elemID - DOM Object ID to show the Countdown
 *
 */
function countdown(duration, elemID) {
  
  var start = Date.now( );
  var difference, hours, minutes, seconds;
  var elem = document.getElementById( elemID );

  function innerTimer( ) {

    difference = duration - ( ( ( Date.now( ) - start ) / 1000 ) | 0 );
    hours = ( difference / 3600 ) | 0;
    minutes = ( (difference % 3600 ) / 60) | 0;
    seconds = ( difference % 60 ) | 0;

    hours = ( hours < 10 ) ? '0' + hours : hours;
    hours = hours + ':';

    minutes = minutes < 10 ? '0' + minutes : minutes;
    minutes = minutes + ':';

    seconds = seconds < 10 ? '0' + seconds : seconds;

    elem.innerHTML = hours + minutes + seconds;

    if ( difference <= 0 ) {
        start = Date.now( ) + 1000;
    }
    
  }

  innerTimer( );
  setInterval( innerTimer, 1000 );

}