function _determineFirstMonthDate(oDate){

		var oFirstDate = new CalendarDate(oDate, this._getPrimaryCalendarType());
		oFirstDate.setDate(1);

		var iMonths = _getMonths.call(this); // to use validation
		if (iMonths <= 12) {
			// only if intervals fit into a year -> otherwise just display the months according to the date
			var iMonth = oDate.getMonth();
			if (12 % iMonths > 0 && iMonth + iMonths > 11) {
				// do not show months over year borders if possible
				iMonth = 12 - iMonths;
			}
			oFirstDate.setMonth(iMonth);
		}

		return oFirstDate;

	}
@param {sap.ui.unified.calendar.CalendarDate} oDate The date, which first date of month will be determined
@returns {sap.ui.unified.calendar.CalendarDate} The first date of the month
@private