timer.js

function Timer(interval, repeat, callback) {
    this.isStopped = true;
    this.interval = interval;
    this.repeat = repeat;
    this.callback = callback;
    this.prevTime = null;

    this.start = function() {
        this.isStopped = false;
    }

    this.stop = function() {
        this.isStopped = true;
        this.prevTime = Date.now();
    }

    this.onTick = function() {
        if (this.isStopped)
            return;

        var curTime = Date.now();

        if (curTime - this.prevTime >= this.interval) {
            if (this.callback) this.callback();
            this.prevTime = curTime;
        }

        if (!this.repeat)
            this.isStopped = true;        
    }
}

var timer = new Timer(1000, true, function() {
    // do stuff
});

timer.start();

API.onUpdate.connect(function() {
    timer.onTick();
});