Written for http://aubreypwd.com/retain-creation-date-posts-never-published/

keep-creation-date-on-draft.php

function ihcrs_wp_insert_post_data( $post_data, $postarr ) {

	// Only on events (I have a function to do this...)
	if ( ! ihcrs_is_event( $postarr['ID'] ) ){
		return $post_data;
	}

	// Get the current posts's data.
	$previous_data = get_post( $postarr['ID'] );

	// If it's not a new post.
	if ( $previous_data ) {

		// Always force the date to remain the same.
		$post_data['post_date'] = $previous_data->post_date;
		$post_data['post_date_gmt'] = $previous_data->post_date_gmt;
	}

	return $post_data;
}

add_action( 'wp_insert_post_data', 'ihcrs_wp_insert_post_data', 99, 2 );