start(time) {
    // now it's a good time to start
    add(this);
    this.playing = true;

    this._startTime = typeof time !== 'undefined' ? time : KEC.Time();
    this._startTime += this._delay;

    if (!this._startFired) {
      if (this._onStart) {
        this._onStart.call(this);
      }

      queueStart.call(this);

      this._startFired = true;
    }

    if (!Tick) Ticker();
    return this;
  }
Starts tweening
@param {number?} time the tween start time
@returns {TweenBase} this instance