Restrict event range for subscriber users

1) Off hand, I don’t know of a way to do that. You could use the filter as you mentioned, but I think it would be better to prevent submission of invalid dates as well, and I’m not sure of a way to do that, right now – I’ll have to look into it. I’m traveling right now, so this may take a little while to look into.

2) There are no filters on the admin table to change the information in that field. It might be possible to use a core filter on the author object, but I don’t know for sure whether any such filter exists that would work for you.

Thank you very much for the tips! I’ll check it up and return with a code in case of I find an answer!

I wish you a nice trip, Joe!

I finally found out how to do it. I just wouldn’t know how to return to the plugin that the post was invalidated. So I just did a wp_die with a message for my client and it worked fine.
I would like to thank you again very much, Joe, for show me the path…

function my_calendar_30_days( $post, $action, $i ) {

  if (!is_user_logged_in()){
    wp_die();
  }

	if ( $action == 'add' || $action == 'edit' || $action == 'copy' ) {
		$begin	= strtotime(trim( $post['event_begin'][ $i ] ));
		$end	= strtotime(trim( $post['event_end'][ $i ] ));
		$now	= time();
		$begindiff	= abs($now - $begin);
		$enddiff	= abs($now - $end);
		$beginInDays= floor($begindiff/(60*60*24));
        if ($end==''){
            $endInDays = 0;
        } else {
			$endInDays	= floor($enddiff/(60*60*24));
        }
		if($beinInDays > 30 || $endInDays > 30){
			//More than 30 days. cancel the event.
            wp_die("<h3>It's not <b>alloewd</b> insert events with more than 30 days from now.</h3>");
		} else {
			//Everything it's ok, return to plugin for other validations
			return $post;
		}
	} else {
//Maybe there's a 'delete' operation?
	  return $post;
	}

}
add_filter( 'mc_pre_checkdata', 'my_calendar_30_days', 10, 3 );