Answer to Stack Overflow's Question #7422487

so7422487.php

<?php
/*
 * Plugin Name: Answer to Stack Overflow's Question #7422487
 * Plugin URI: http://stackoverflow.com/questions/7422487/wordpress-urls-for-categories-and-years
 * Description: :)
 * Author: Federico Cingolani
 * Version: 1.0.0
 * Author URI: http://twitter.com/fcingolani
 */


register_activation_hook( __FILE__, 'flush_rewrite_rules' );
register_deactivation_hook( __FILE__, 'flush_rewrite_rules' );


function so7422487_add_rewrite_rules( $wp_rewrite ) {
	$new_rules = array( 
		'(.+)/(.+)' => 'index.php?category_name='.$wp_rewrite->preg_index(1).'&year='.$wp_rewrite->preg_index(2) );

	$wp_rewrite->rules = $new_rules + $wp_rewrite->rules;
}

add_action('generate_rewrite_rules', 'so7422487_add_rewrite_rules');