Denies access to any content that was published before the user's registration date

wpf-restrict-past-content.php

<?php

//
// Denies access to any content that was published before the user's registration date
//
function disallow_before_date_published( $can_access, $user_id, $post_id ) {

	$published = get_the_date( 'U', $post_id );
	$userdata  = get_userdata( $user_id );

	if ( strtotime( $userdata->user_registered ) < $published ) {
		$can_access = false;
	}

	return $can_access;

}

add_filter( 'wpf_user_can_access', 'disallow_before_date_published', 10, 3 );

//
// OR: Do it based on the date a tag was appplied
//

function disallow_before_date_published_alt( $can_access, $user_id, $post_id ) {

	$published = get_the_date( 'U', $post_id );
	$startdate = get_user_meta( $user_id, 'startdate', true );

	// If the content was published before the user's startdate field, deny access

	if ( empty( $startdate ) || $startdate < $published ) {
		$can_access = false;
	}

	return $can_access;

}

add_filter( 'wpf_user_can_access', 'disallow_before_date_published_alt', 10, 3 );

function update_tag_applied_timestamp( $user_id, $tags ) {

	// When TAGNAME is applied, save the current time to the startdate field

	if ( in_array( 'TAGNAME', $tags ) ) {
		update_user_meta( $user_id, 'startdate', time() );
	}

}

add_action( 'wpf_tags_applied', 'update_tag_applied_timestamp', 10, 2 );