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Thank you very much for your time. I made several test and I don't know why it does not work without "target". (maybe cache on the server, or on the cdn?).

By the way, I created a button "add to google calendar" as well. It can be useful as well. I share the code below if it can help our community:

First, include in the function.php (or in a php code snippet):

function addToGoogleCalendar($title='', $startdate='', $enddate='', $location='', $details='')
{
    $startdate = ($startdate ? $startdate : time());
    $startdate = (is_numeric($startdate) ? $startdate : strtotime($startdate));
    $enddate = ($enddate ? $enddate : $startdate + 3600);
    $enddate = (is_numeric($enddate) ? $enddate : strtotime($enddate));   
    $google_url = "https://www.google.com/calendar/event";
    $action = "?action=TEMPLATE";
    $title = ( $title ? ("&text=" . urlencode($title)) : "") ;
    $dates = "&dates=" . getIcalDate($startdate) . "Z/" . getIcalDate($enddate) . "Z";
    $location = ( $location ? ("&location=" . urlencode($location)) : "") ;
    $details = ( $details ? ("&details=" . urlencode($details)) : "") ;
    $out = $google_url . $action . $title . $dates . $location . $details;

    return $out;
}
function getIcalDate($time, $incl_time = true)
{
    return $incl_time ? date('Ymd\THis', $time) : date('Ymd', $time);
}

Then create a shortcode [googlecalendar]

<?php
add_shortcode( 'googlecalendar', function() {
    $start_date = rwmb_get_value( 'ci_event_start_date', array( 'format' => 'Y-m-d g:iA' ) );
    $start_date = wp_date( 'Ymd\THis', $start_date );
    $end_date = rwmb_get_value( 'ci_event_end_date', array( 'format' => 'Y-m-d g:iA' ) );
    $end_date = wp_date( 'Ymd\THis', $end_date );
    $title = get_the_title() ;
    $location = rwmb_meta( 'ci_event_location_address' ) ;
        $urlgc = addToGoogleCalendar($title, $start_date, $end_date, $location, $title);
     
return $urlgc;
}
);
?>

The result of the shortcode is a link with all the data of my custom post "events) to create an event in google calendar.

Hello.
I don't see how to solve that. Once the "form" is submitted, it stays on a blank page.

Everytging seems to work. The ics file whiich is created is correct. My onlyissue is that I odn't know why at the end of the process, the visitor is blocked on a blanck page.

Here are my codes:

<?php

class ICS {
  const DT_FORMAT = 'Ymd\THis';

  protected $properties = array();
  private $available_properties = array(
    'description',
    'dtend',
    'dtstart',
    'location',
    'summary',
    'url'
  );

  public function __construct($props) {
    $this->set($props);
  }

  public function set($key, $val = false) {
    if (is_array($key)) {
      foreach ($key as $k => $v) {
        $this->set($k, $v);
      }
    } else {
      if (in_array($key, $this->available_properties)) {
        $this->properties[$key] = $this->sanitize_val($val, $key);
      }
    }
  }

  public function to_string() {
    $rows = $this->build_props();
    return implode("\r\n", $rows);
  }

  private function build_props() {
    // Build ICS properties - add header
    $ics_props = array(
      'BEGIN:VCALENDAR',
      'VERSION:2.0',
      'PRODID:-//hacksw/handcal//NONSGML v1.0//EN',
      'CALSCALE:GREGORIAN',
      'BEGIN:VEVENT'
    );

    // Build ICS properties - add header
    $props = array();
    foreach($this->properties as $k => $v) {
      $props[strtoupper($k . ($k === 'url' ? ';VALUE=URI' : ''))] = $v;
    }

    // Set some default values
    $props['DTSTAMP'] = $this->format_timestamp('now');
    $props['UID'] = uniqid();

    // Append properties
    foreach ($props as $k => $v) {
      $ics_props[] = "$k:$v";
    }

    // Build ICS properties - add footer
    $ics_props[] = 'END:VEVENT';
    $ics_props[] = 'END:VCALENDAR';

    return $ics_props;
  }

  private function sanitize_val($val, $key = false) {
    switch($key) {
      case 'dtend':
      case 'dtstamp':
      case 'dtstart':
        $val = $this->format_timestamp($val);
        break;
      default:
        $val = $this->escape_string($val);
    }

    return $val;
  }

  private function format_timestamp($timestamp) {
    $dt = new DateTime($timestamp);
    return $dt->format(self::DT_FORMAT);
  }

  private function escape_string($str) {
    return preg_replace('/([\,;])/','\\\$1', $str);
  }
}

function justread_ics_download() {

        if ( isset( $_GET['ics'] ) ) {

               // include get_stylesheet_directory() . '/inc/ICS.php';

                header('Content-Type: text/calendar; charset=utf-8');

                header('Content-Disposition: attachment; filename=invite.ics');

                $ics = new ICS(array(

                       'location' => $_POST['location'],

                       'dtstart' => $_POST['start_date'],

                       'dtend' => $_POST['end_date'],

                       'summary' => $_POST['summary'],

                 ));
                //return "It works";

                echo $ics->to_string();

                
                exit();

                return;

        }

}

add_action('template_redirect', 'justread_ics_download' );
?>

And the shortcode:

<?php
add_shortcode( 'calendardata', function() {
    $start_date = rwmb_get_value( 'ci_event_start_date', array( 'format' => 'Y-m-d g:iA' ) );
    $start_date = wp_date( 'Ymd\THis', $start_date );
    $end_date = rwmb_get_value( 'ci_event_end_date', array( 'format' => 'Y-m-d g:iA' ) );
    $end_date = wp_date( 'Ymd\THis', $end_date );
        echo $end_date;
        echo $start_date;
?>
    <form method="post" action="?ics=true">
        <input type="hidden" name="start_date" value="<?php echo $start_date; ?>">
        <input type="hidden" name="end_date" value="<?php echo $end_date; ?>">
        <input type="hidden" name="location" value="<?php echo rwmb_meta( 'ci_event_location_address' ); ?>">
        <input type="hidden" name="summary" value="<?php the_title(); ?>">
        <input type="submit" value="Add to Calendar">
    </form>
<?php
return;
}
);
?>

Thank you very much.

Thank you.

Hi,
Thank you for your answer.
It's what I tried to use with sanitize_text_field( $datesofevent ) in my code. Is it correct?

Thank you very much.
Now it works like that:

<?php
add_shortcode( 'datesofevent', function() {

    $post_id = get_the_ID();
    // Get the dates
       $startdate = get_post_field( 'ci_event_start_date', $post_id );
       $enddate = get_post_field( 'ci_event_end_date', $post_id );
       echo "from ", date('l jS \of F Y ', $startdate), " to ", date('l jS \of F Y ', $enddate);
    
// return
    return ;
}
             );
             ?>

I wanted to write :
return "from ". date('l jS \of F Y ', $startdate). " to ". date('l jS \of F Y ', $enddate);

but it didn't work. Si I let return alone and included a "echo xxxxx"

Sorry but I have a new question related to the shortcodes. Do I need to sanitize the data in the shortcode. I thought they were already sanitized in the metaboxes.
If yes, I understand that I have to do it for every shortcode or function I have created? Thank you for your clarification.