JSFiddle - React, Tailwind, and code Playground

JavaScript

$(function(){
     /** 
     * @param {int} month 0-11 based
     * @param {int} year 4 digits based
     * @returns {int} number of days
     */
    function daysInMonth(month, year) {
        return new Date(year, month + 1, 0).getDate(); //adding 1 month, and zero day will be the previous month's last day
    }

    /**
     * @param {int} The month number, 0 based
     * @param {int} The year, not zero based, required to account for leap years
     * @return {Date[]} List with date objects for each day of the month
     */
    function getDaysInMonth(month, year) {
        var date = new Date(year, month, 1);
        var days = [];
        while (date.getMonth() === month) {
            days.push(new Date(date));
            date.setDate(date.getDate() + 1);
        }
        return days;
    }
     /**
     * @param {int} currentMonth 0-11 
     * @param {int} currentYear 4 digit year
     * @return {Date[]} List with date objects of previous other month
     */
    function getDatePrevOtherMonth(currentMonth, currentYear) {
        var indexCurrentFirstDay = new Date(currentYear, currentMonth, 1).getDay();
//console.log(indexCurrentFirstDay);
        var pYear = "";
        var pMonth = "";
        var pDate = [];
        if (currentMonth === 0) {
            pYear = currentYear - 1;
            pMonth = 11;
        } else {
            pMonth = currentMonth - 1;
            pYear = currentYear;
        }
        var totalDaysOfPreviousMonth = daysInMonth(pMonth, pYear);
        if (indexCurrentFirstDay > 0) {
            var prevMonthDayStart = totalDaysOfPreviousMonth - (indexCurrentFirstDay - 1); //-1 to ensure inclusive day
//    console.log(prevMonthDayStart);
            for (var i = prevMonthDayStart; i <= totalDaysOfPreviousMonth; i++) {

                pDate.push(new Date(pYear, pMonth, i));
//console.log(pYear + "-" + pMonth + "-" + i);
            }
        }

        return pDate;
    }

    /**
     * 
     * @param {int} currentMonth 0-11 
  ...