Source Code for https://formidable-masterminds.com/date-math-demo/ form

date-math-demo.js

<script>
/* install this script in the after fields section on the
 * form's CustomizeHTML page.
 */
jQuery(document).ready(function($) {
    "use strict";
    /**
     * original code found on StackExchange
     * addmonths:  function used to calculate renewal date
     * date {dateStr}
     * months {numeric}
     * returns new Date object
     */  
    function addMonths(date, months) {
        var d = date.getDate();
        date.setMonth(date.getMonth() + +months);
        if (date.getDate() != d) {
            date.setDate(0);
        }
        return date;
    }
    
    /* clears renewal date */
    function clear_renewal_date_val() {
        $("#field_dtmthdemo_renewal_date").val("");
    }
    
    /* Working jQuery */
    /* multi-field listener for on change event */
    $("#field_dtmthdemo_purchase_date, #field_dtmthdemo_purchase_type, #field_dtmthdemo_subscription_type").on("change", function() {
        /* if transaction is a subscription, continue processing renewal date calculation */
        if ( $("#field_dtmthdemo_purchase_type").val() == "Subscription" ) {
            /* #field_dtmthdemo_subscription_type is a separate value dropdown
             * the value is passed as the months parameter to addmonths()
             */
            if ( $("#field_dtmthdemo_subscription_type").val() !== "" ) {
                /* the magic */
                var renewal_factor = $("#field_dtmthdemo_subscription_type").val(), // separate value dropdown value
                    calc_date = new Date( $("#field_dtmthdemo_purchase_date").val() ), // convert the purchase date value from dateStr to date object
                    renewal_date = addMonths(calc_date, renewal_factor ); // retrieve the renewal date object
                /* convert the renewal date Date object to a dateStr and write results to renewal date value */
                
                $("#field_dtmthdemo_renewal_date").val($.datepicker.formatDate('mm/dd/yy', renewal_date) );
            } else {
                clear_renewal_date_val();
            }
        } else {
            clear_renewal_date_val();
        }
    });
});
</script>