bp_get_user_last_activity( int $user_id )

function bp_get_user_last_activity( $user_id = 0 ) {
	$activity = '';

	$last_activity = BP_Core_User::get_last_activity( $user_id );
	if ( ! empty( $last_activity[ $user_id ] ) ) {
		$activity = $last_activity[ $user_id ]['date_recorded'];
	}

	/**
	 * Filters the last activity for a given user.
	 *
	 * @since BuddyPress 1.9.0
	 *
	 * @param string $activity Time of last activity, in 'Y-m-d H:i:s' format or
	 *                         an empty string if none found.
	 * @param int    $user_id  ID of the user being checked.
	 */
	return apply_filters( 'bp_get_user_last_activity', $activity, $user_id );
}