timer.js

(function(window, document){

    'use strict';

    var $input = document.querySelector('[data-js="input"]');
    var $start = document.querySelector('[data-js="start"]');
    var $stop = document.querySelector('[data-js="stop"]');
    var $reset = document.querySelector('[data-js="reset"]');
    var interval;

    $start.addEventListener('click', startTimer, false);

    function startTimer() {
        $input.value = +$input.value + 1;
        setTimeout(startTimer, 1000);
    }
    
})(window, document);