<?php
function rewrite_rule_by_date() {
    add_rewrite_rule('(by\-date)/(\[0-9]{4}-[0-9]{2}-[0-9]{2})$', 'index.php?pagename=$matches[1]&wpem_event_date=$matches[2]', 'top');
}
add_action( 'init', array( &$this, 'rewrite_rule_by_date' ) );

function query_var_by_date( $vars ) {
    add_rewrite_tag( '%wpem_event_date%', '([0-9]{4}-[0-9]{2}-[0-9]{2})');
}
add_action( 'query_vars', array( &$this, 'query_var_by_date' ) );

function flush_rules() {
    global $wp_rewrite;

    $wp_rewrite->flush_rules();
}
register_activation_hook( __FILE__, array( &$this, 'flush_rules' ) );