OnOptionsetChange: Get OptionSet value, add N days + the days until the end of the month and set to another field

dynamics365

function SetDueDate(executionContext) {
    var obj = Xrm.Page.getAttribute("paymenttermscode");
    if (obj != null) {
        var created_date = Xrm.Page.getAttribute("createdon");
        if (created_date != null) {
            var optionSetValue = obj.getValue();
            if (optionSetValue == "100000000") {
                var new_date = addDays(created_date.getValue() ,60);
                new_date = addDays(new_date,getMonthDaysLeft(new_date));
                Xrm.Page.getAttribute("duedate").setValue(new_date);
            } else if (optionSetValue == "100000001") {
                var new_date = addDays(created_date.getValue() ,45);
                new_date = addDays(new_date,getMonthDaysLeft(new_date));
                Xrm.Page.getAttribute("duedate").setValue(new_date);
            } else if (optionSetValue == "100000002") {
                Xrm.Page.getAttribute("duedate").setValue(created_date.getValue());
            }
        }
    }
}
function addDays(date, days) {
var result = new Date(date);
result.setDate(result.getDate() + parseInt(days));
return result;
}
function getMonthDaysLeft(date){
    return new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate() - date.getDate();
}