Timer.js

function Timer(onTick) {
    var self = this,
        interval,
        startTime,
        count
    self.start = function() {
        count = 0
        startTime = (new Date()).toISOString()
        interval = window.setInterval(function() {
            count++
            onTick(count)
        }, 1000)
    }
    self.stop = function() {
        count = 0;
        clearInterval(interval)
        return {
            start: startTime,
            end: (new Date()).toISOString()
        }
    }
}