Add business days to date jquery

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jQuery

jQuery-UI

I have an input box where users can enter a number , positive and negative, to calculate either business or Calendar days. I am using jquery to calculate the days and it works when the number entered is a positive number, for example (10) but does not work when the number entered is negative for example (-10) . Below is my code. Any help will be appreciated

What I have tried:

function AddBusinessDays(startDate, workDaysToAdd) {
    
    var curDate = new Date(startDate);
    var realDaysToAdd = 0;
    while (workDaysToAdd > 0) {
        realDaysToAdd++;
        if (IsWorkDay(curDate)) {
            workDaysToAdd--;
        }
        curDate.setDate(curDate.getDate() + 1);
    }
     
    return realDaysToAdd;
}

function IsWorkDay(date) {
    var curDate = new Date(date);
    var dayOfWeek = curDate.getDay();
    var isWorkDay;
    if (dayOfWeek === 0 || dayOfWeek === 5) {
        isWorkDay = false;
    }
    else {
        isWorkDay = !isNationalDay(curDate);
    }
    return isWorkDay;
}

function isNationalDay(date) {
    var thisDate = new Date(date);
    var thisMoment = moment(thisDate);
    var holiday = thisMoment.holiday();
    var isHoliday = holiday != 'undefined' && holiday != null && holiday.length > 0;
    return isHoliday;
}