code: function(now) {
  
  		if (now < this._end_time) {
  
  			this._callAnimators(this._transition((now - this._started_time) / this._duration));
  
  		} else {
  
  			this._callAnimators(this._transition(1));
  			this._finish();
  
  		}
  
  	}
docs: Perform animation in normal direction
  @param {number} now The current global time in milliseconds