Как сделать рабочий таймер обратного отсчёта в новостях через доп поля?

<style>
   .timer {margin:50px 50px;font-size:30px;text-align: center;}
</style>
   <p class="timer">
      <span id="days"></span>
      <span id="hours"></span>
      <span id="minutes"></span>
      <span id="seconds"></span>
   </p>
<script>
   function countdown(dateEnd) {
   var timer, days, hours, minutes, seconds;
   
   dateEnd = new Date(dateEnd);
   dateEnd = dateEnd.getTime();
   
   if ( isNaN(dateEnd) ) {
   return;
   }
   
   timer = setInterval(calculate, 1000);
   
   function calculate() {
   var dateStart = new Date();
   var dateStart = new Date(dateStart.getUTCFullYear(),
                            dateStart.getUTCMonth(),
                            dateStart.getUTCDate(),
                            dateStart.getUTCHours(),
                            dateStart.getUTCMinutes(),
                            dateStart.getUTCSeconds());
   var timeRemaining = parseInt((dateEnd - dateStart.getTime()) / 1000)
   
   if ( timeRemaining >= 0 ) {
     days    = parseInt(timeRemaining / 86400);
     timeRemaining   = (timeRemaining % 86400);
     hours   = parseInt(timeRemaining / 3600);
     timeRemaining   = (timeRemaining % 3600);
     minutes = parseInt(timeRemaining / 60);
     timeRemaining   = (timeRemaining % 60);
     seconds = parseInt(timeRemaining);
   
     document.getElementById("days").innerHTML    = parseInt(days, 10);
     document.getElementById("hours").innerHTML   = ("0" + hours).slice(-2);
     document.getElementById("minutes").innerHTML = ("0" + minutes).slice(-2);
     document.getElementById("seconds").innerHTML = ("0" + seconds).slice(-2);
   } else {
     return;
   }
   }
   
   function display(days, hours, minutes, seconds) {}
   }
   countdown('[xfvalue_day]/[xfvalue_month]/[xfvalue_year] [xfvalue_hours]:[xfvalue_minutes]:[xfvalue_seconds] AM');
</script>