Таймер, который считает вперед

<style>
    html,
    body {
      margin: 0;
      padding: 0;
    }
 
    #header {
      text-align: center;
      font-size: 20px;
      width: 100%;
      height: 50px;
      line-height: 50px;
      background: white;
      border-bottom: 1px solid black;
    }
 
    #content {
      text-align: center;
      top: 25px;
      position: absolute;
      overflow: auto;
      line-height: 25px;
    }
 
  </style>
 
<div id="header">
      <span id="counter1"><strong>16</strong> дней </span>
</div>
<div id="content"></div>
 
<script>
function Counter(initDate, id){
    this.counterDate = new Date(initDate);
    this.countainer = document.getElementById(id);
    this.numOfDays = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ];
    this.borrowed = 0, this.years = 0, this.months = 0, this.days = 0;
    this.hours = 0, this.minutes = 0, this.seconds = 0;
    this.updateNumOfDays();
    this.updateCounter();
}
 
Counter.prototype.updateNumOfDays=function(){
    var dateNow = new Date();
    var currYear = dateNow.getFullYear();
    if ( (currYear % 4 == 0 && currYear % 100 != 0 ) || currYear % 400 == 0 ) {
        this.numOfDays[1] = 29;
    }
    var self = this;
    setTimeout(function(){self.updateNumOfDays();}, (new Date((currYear+1), 1, 2) - dateNow));
}
 
Counter.prototype.datePartDiff=function(then, now, MAX){
    var diff = now - then - this.borrowed;
    this.borrowed = 0;
    if ( diff > -1 ) return diff;
    this.borrowed = 1;
    return (MAX + diff);
}
 
Counter.prototype.calculate=function(){
    var futureDate = this.counterDate > new Date()? this.counterDate : new Date();
    var pastDate = this.counterDate == futureDate? new Date() : this.counterDate;
    this.seconds = this.datePartDiff(pastDate.getSeconds(), futureDate.getSeconds(), 60);
    this.minutes = this.datePartDiff(pastDate.getMinutes(), futureDate.getMinutes(), 60);
    this.hours = this.datePartDiff(pastDate.getHours(), futureDate.getHours(), 24);
    this.days = this.datePartDiff(pastDate.getDate(), futureDate.getDate(), this.numOfDays[futureDate.getMonth()]);
    this.months = this.datePartDiff(pastDate.getMonth(), futureDate.getMonth(), 12);
    this.years = this.datePartDiff(pastDate.getFullYear(), futureDate.getFullYear(), 0);
}
 
Counter.prototype.addLeadingZero=function(value){
    return value < 10 ? ("0" + value) : value;
}
 
Counter.prototype.formatTime=function(){
    this.seconds = this.addLeadingZero(this.seconds);
    this.minutes = this.addLeadingZero(this.minutes);
    this.hours = this.addLeadingZero(this.hours);
}
 
Counter.prototype.updateCounter=function(){
    this.calculate();
    this.formatTime();
    this.countainer.innerHTML = "Прошел" + " <strong>" + this.days + "</strong> " + (this.days == 1? "день" : "дня");
    var self = this;
    setTimeout(function(){self.updateCounter();}, 1000);
}
</script>
 
<script type="text/javascript">
  new Counter('September 25, 2018 00:00:00', 'counter1');
  new Counter(((new Date()).getTime()+10000), 'counter2');
</script>