from-to-calculator.php

<?php

add_action('wp_footer', function () {

    global $post;
    if (!$post instanceof WP_Post || !has_shortcode($post->post_content, 'forminator_form')) {
        return;
    }
?>
    <script type="text/javascript">
        jQuery(document).ready(function($) {
            setTimeout(function() {
                $('.forminator-custom-form').trigger('after.load.forminator');
            });

            $(document).on('after.load.forminator', function(event, form_id) {

                let _form_id = 105,
                    _date_field_id1 = 'date-1',
                    _date_field_id2 = 'date-2',
                    _days_field_id = 'number-1', //or enter text-field id
                    _separated_date = '/',
                    _form = $('#forminator-module-' + _form_id);
                if (_form.length) {
                    let _date_field1 = _form.find('[name="' + _date_field_id1 + '"]'),
                        _date_field2 = _form.find('[name="' + _date_field_id2 + '"]');
                    // 		_default_value1 = _date_field1.val(),
                    // 		_default_value2 = _date_field1.val();

                    _date_field1.on('change', function() {
                        let _value = $(this).val();
                        if (_value) {
                            _date_field2_value = _date_field2.val();
                            if (_date_field2_value) {
                                calc_days(_value.split(_separated_date), _date_field2_value.split(_separated_date), $(this).data('format').split(_separated_date));
                            }
                        }
                    });

                    _date_field2.on('change', function() {
                        let _value = $(this).val();
                        if (_value) {
                            _date_field1_value = _date_field1.val();
                            if (_date_field1_value) {
                                calc_days(_date_field1_value.split(_separated_date), _value.split(_separated_date), $(this).data('format').split(_separated_date));
                            }
                        }
                    });


                    function calc_days(_date_value_1, _date_value_2, _date_format) {
                        let _date1 = {},
                            _date2 = {};
                        for (let _t in _date_format) {
                            _date1[_date_format[_t]] = _date_value_1[_t];
                            _date2[_date_format[_t]] = _date_value_2[_t];
                        }

                        // Fill age field.
                        _date1 = new Date(_date1['yy'], _date1['mm'], _date1['dd']);
                        _date2 = new Date(_date2['yy'], _date2['mm'], _date2['dd']);
                        _form.find('[name="' + _days_field_id + '"]').val(Math.round(Math.abs(_date2 - _date1) / 86400000));
                    }
                }
            });
        });
    </script>
<?php
}, 21);