javascript 完全個人用

		var year = 2017;
		var month = 4;
		var startDayOfWeek = START_SUNDAY;
		var week = 6;
		/*
		if(!year.matches(^\d{4}$))) return;
		if(!month.matches(^\d*$)) return;
		if(!(startDayOfWeek == START_SUNDAY) || (startDayOfWeek == START_MONDAY))) return;
		*/
		console.log(year + "年" + month + "月は" + countWeekOfMonth(year, month, startDayOfWeek) + "週です。");
		console.log(year + "年" + month + "月(開始曜日 : " + startDayOfWeek + ")の、週開始日は" +  getFirstDateOfNthWeekOfMonth(year, month, startDayOfWeek, week) + "日です");

		// monthは日本式  1月 → month=1
		function countWeekOfMonth(year, month, startDayOfWeek) {
			var daysOfMonth = new Date(year, month, 0).getDate();
			var day = new Date(year, month-1, 1).getDay(); // 今月の1日の曜日

			var weekCount = 1;
			for(var i=1; i<daysOfMonth; ++i){// 今月の2日目からカウント
				if((day+i)%7 == startDayOfWeek) weekCount++;
			}

			return weekCount;
		}

		function getFirstDateOfNthWeekOfMonth(year, month, startDayOfWeek, n) {
			if(n == 1){
				return new Date(year, month-1, 1);
			}
			var daysOfMonth = new Date(year, month, 0).getDate();
			var day = new Date(year, month-1, 1).getDay(); // 今月の1日の曜日

			var weekCount = 1;
			for(var i=1; i<daysOfMonth; ++i){// 今月の2日目からカウント
				if((day+i)%7 == startDayOfWeek) weekCount++;
				if(weekCount == n) return new Date(year, month-1, i+1).getDate();
			}

		}

		function daysOfMonth(year, month) {
			return new Date(year, month, 0).getDate();
		}