A countdown timer in JS with AddTime();

countdown.js

/********************************************************************************************************************
Countdown.js is a simple snippet to add a countdown timer for your app.
I've forked this coding from https://gist.github.com/adhithyan15/4350689 which was
originally a fork of http://jsfiddle.net/HRrYG/.
I've implemented a new feature, addTime(); and modified the coding a little bit to be optimized for usage in my app.
You can see my demo here https://jsfiddle.net/gounshin/as6drbyL/1/
Since the original code that I forked was released under Creative Commons by SA license, so is mine.
********************************************************************************************************************/

function Countdown(seconds) {
  this.sec = seconds;
  this.mins = seconds > 60? seconds/60 : 0;
        
  tick();

  function tick() {
      if(this.sec > 60){
          this.current_minutes = mins-1;
      }
      this.sec--;
      timeView.textContent = (!this.mins? '00' : this.current_minutes.toString()) + ':' + (this.sec < 10 ? '0' : '') + String(this.sec);
      if( this.sec > 0 ) {
          // stopTime id is necessary for the function addTime(); if you are just going to use the function countdown();, you can omit it.
          stopTime = setTimeout(tick, 1000);
      } else {
          if(this.mins > 1){
              countdown(this.mins-1);           
          }
      }
  }
}

function AddTime(seconds){
    /* If extra time is added without clearTimeout(stopTime);, the function tick(); inside of the function countdown(); runs twice.  
       Since the seconds in this parameter is inherited from this.sec in the function countdown(),
       the seconds in the timer are decremented every 2 seconds. More accurately, this.sec--; in the function countdown() is invoked twice without clearTimeout(stopTime);.  
       So the seconds in the timer don't work in the way you would expect. */
	clearTimeout(stopTime);
	countdown.call(this.sec, seconds);
	sec = this.sec + seconds;
}

Countdown(n);
AddTime(n); //where n is the number of seconds required.