function _togglePrevNexYearPicker(){

		var oYearPicker = this._getYearPicker();
		var iYears = oYearPicker.getYears();
		var oDate = CalendarUtils._createUniversalUTCDate(oYearPicker.getProperty("_middleDate").toLocalJSDate());
		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);
		}

	}
Disable / Enable the next and previous button in the Header.
This function assumes there is a "yearPicker" aggregation.
So callers must take care.
@returns {void}
@private