code: function getDatePos(date, month, notValue = null) {
  	const monthPos = (date.month === month.date.month && 'current') ||
  		(date.month === month.nextMonth.date.month && 'next') ||
  		(date.month === month.previousMonth.date.month && 'prev');
  		
  	switch (monthPos) {
  		case 'current':
  			return calculateDatePos(month.startDayOfMonth, date.day);
  		case 'prev':
  			return calculateDatePosinPrev(
  				month.startDayOfMonth,
  				month.previousMonth.daysCount,
  				date.day
  			);
  		case 'next':
  			const posNext = calculateDatePosinNext(
  				month.startDayOfMonth,
  				month.daysCount,
  				date.day
  			);
  
  			return posNext;
  
  		default:
  			return notValue;
  	}
  }
docs: Calculates week and weekday indexes in the month
  
  @param {object} date
  @param {object} month
  @param {object} notValue
  @returns {({weekIndex:number, weekDayIndex:number}|*)}