simple_game_timer.js

function Timer() {
    this.gameTime = 0;
    this.lastTick = 0;

    this.maxStep = 0.05;
    this.lastTimestamp = 0;
}

Timer.prototype.step = function() {
    var current = Date.now();
    var delta = (current - this.lastTimestamp) / 1000;
    this.gameTime += Math.min(delta, this.maxStep);
    this.lastTimestamp = current;
}

Timer.prototype.tickDiff = function() {
    var delta = this.gameTime - this.lastTick;
    this.lastTick = this.gameTime;
    return delta;
}