code: function _togglePrevNexYearPicker(){
  
  		var oYearPicker = this.getAggregation("yearPicker");
  		var iYears = oYearPicker.getYears();
  		var oDate = CalendarUtils._createUniversalUTCDate(oYearPicker.getFirstRenderedDate());
  		oDate.setUTCFullYear(oDate.getUTCFullYear() + Math.floor(iYears / 2));
  		var oHeader = this.getAggregation("header");
  		var oMaxDate = new UniversalDate(this._oMaxDate);
  		oMaxDate.setUTCFullYear(oMaxDate.getUTCFullYear() - Math.ceil(iYears / 2));
  		oMaxDate.setUTCMonth(11, 31);
  		var oMinDate = new UniversalDate(this._oMinDate);
  		oMinDate.setUTCFullYear(oMinDate.getUTCFullYear() + Math.floor(iYears / 2) + 1);
  		oMinDate.setUTCMonth(0, 1);
  
  		if (oDate.getTime() > oMaxDate.getTime()) {
  			oHeader.setEnabledNext(false);
  		} else {
  			oHeader.setEnabledNext(true);
  		}
  		if (oDate.getTime() < oMinDate.getTime()) {
  			oHeader.setEnabledPrevious(false);
  		} else {
  			oHeader.setEnabledPrevious(true);
  		}
  
  	}
docs: Disable / Enable the next and previous button in the Header.
  This function assumes there is a "yearPicker" aggregation.
  So callers must take care.
  @return {void}
  @private