if ( is_admin() ) {
    add_action( 'wp_ajax_dynamic_dropdown', 'dynamic_dropdown_func' );
    add_action( 'wp_ajax_nopriv_dynamic_dropdown', 'dynamic_dropdown_func' );
}
function dynamic_dropdown_func () {
    global $wpdb;
    if (isset($_POST['event'])) {
        $event_id = $_POST['event'];
        $first = get_field('first_day',$event_id);
        $last = get_field('last_day',$event_id);
        $event_dates = '<option value="" disabled selected>Choose Date</option>';
        $event_dates .= '<option value="'.$first.'">'.$first.'</option>';
        while($first<$last) :
            $first = $first + 1;
            $event_dates .= '<option value="'.$first.'">'.$first.'</option>';
        endwhile;
    }
ob_clean();
return $event_dates;
wp_die();
}

<?php function date_chooser () {
    $ajax_url = admin_url( 'admin-ajax.php' );
    $grabDates = "
        <script>
            var ajaxUrl = '{$ajax_url}',
            dropdownEvent = jQuery('#chooseEvent'),
            dropdownDate  = jQuery('#chooseDate');
            dropdownEvent.on('change', function (e) {
                var value = e.target.selectedOptions[0].value,
                success,
                data;
                if (!!value) {
                    data = {
                        'event' : value,
                        'action' : 'dynamic_dropdown'
                    };
                    success = function ( response ) {
                        dropdownDate.html( response );
                    };
                    jQuery.post( ajaxUrl, data, success );
                }
            });
        </script>";
    return $grabDates;
}
echo date_chooser(); ?>