bbpress-add-date-created-on.php

<?php
add_action( 'bbp_theme_after_topic_started_by', function() {
	$topic = bbp_get_topic( bbp_get_topic_id() );

	if ( ! $topic || ! is_a( $topic, 'WP_Post' ) ) {
		return;
	}

	try {
		$date_created = date_create( $topic->post_date )->format( get_option( 'date_format' ) );
		print ' on ' . esc_html( $date_created );
	}
	catch ( Exception $e ) {}
} );