calculate # of weeks you can spend in a trip given that you have two months vacations, starting on the 1st day of the first month and finishing on the last day of the second month. you can leave only on mondays and returns only on sundays

vacationCalendar.js

function solution(Y, A, B, W) {
    // write your code in JavaScript (Node.js 8.9.4)
    const MONTH_DAYS = {
    	"January":31, "February":28, "March":31, "April":30, "May":31, "June":30, "July":31, "August":31, "September":30, "October":31, "November":30, "December":31
    };
    const MONTHS = {
    	"January":0, "February":1, "March":2, "April":3, "May":4, "June":5, "July":6, "August":7, "September":8, "October":9, "November":10, "December":11
    };

    function findFirstVacationMonday(year, month) {
	       let monday = new Date(year, MONTHS[month], 1, 0, 0, 0, 0);
	       while(monday.getDay() !== 1){
	       		monday.setDate(monday.getDate()+1);
	       }
	       return monday;
	}

	function findLastVacationSunday(year, month) {
		   let lastDayOfMonth = MONTH_DAYS[month];
		   if (month === "February") {
		   	 if (year % 4 === 0) {
		   	 	lastDayOfMonth++;
		   	 }
		   }
		   let sunday = new Date(year, MONTHS[month], lastDayOfMonth, 0, 0, 0, 0);
	       while(sunday.getDay() !== 0){
	       		sunday.setDate(sunday.getDate()-1);
	       }
	       return sunday;
	}

	function countDays(firstMonday, lastSunday) {
		return Math.round((lastSunday - firstMonday) / (24 * 60 * 60 * 1000))+1
	}

	let monday = findFirstVacationMonday(Y, A);
	let sunday = findLastVacationSunday(Y, B);

	return countDays(monday, sunday) / 7;
}