Wordpress3.1のデフォルトで組み込まれているウィジェットの「最近の投稿（サイトの最新の投稿 ）」に日付を付加しプラグイン化したもの。wp-content\plugins\wp-recentpost-info\wp-recentpost-info.phpに保存してプラグインを有効にしてウィジェットで「最近の投稿(custom)」がそれ。

wp-recentpost-info.php

<?php
/*
Plugin Name: Recent post info
Plugin URI: 
Description: Display post info.
Version: 0.1
Author: cherenkov
Author URI: http://d.hatena.ne.jp/Cherenkov/
*/



class WP_Widget_RecentPostInfo extends WP_Widget {

	function WP_Widget_RecentPostInfo() {
		$widget_ops = array('classname' => 'widget_recent_entries', 'description' => __( "The most recent posts on your site").'. 日付表示' );
		$this->WP_Widget('recent-posts2', __('Recent Posts').'(custom)', $widget_ops);
		$this->alt_option_name = 'widget_recent_entries';

		add_action( 'save_post', array(&$this, 'flush_widget_cache') );
		add_action( 'deleted_post', array(&$this, 'flush_widget_cache') );
		add_action( 'switch_theme', array(&$this, 'flush_widget_cache') );
	}

	function widget($args, $instance) {
		$cache = wp_cache_get('widget_recent_posts', 'widget');

		if ( !is_array($cache) )
			$cache = array();

		if ( isset($cache[$args['widget_id']]) ) {
			echo $cache[$args['widget_id']];
			return;
		}

		ob_start();
		extract($args);

		$title = apply_filters('widget_title', empty($instance['title']) ? __('Recent Posts') : $instance['title'], $instance, $this->id_base);
		if ( ! $number = absint( $instance['number'] ) )
 			$number = 10;

		$r = new WP_Query(array('posts_per_page' => $number, 'nopaging' => 0, 'post_status' => 'publish', 'ignore_sticky_posts' => true));
		if ($r->have_posts()) :
?>
		<?php echo $before_widget; ?>
		<?php if ( $title ) echo $before_title . $title . $after_title; ?>
		<ul>
		<?php  while ($r->have_posts()) : $r->the_post(); ?>
		<li class="newslist"><?php the_time('Y年n月j日'); ?><ul><li><a href="<?php the_permalink() ?>" title="<?php echo esc_attr(get_the_title() ? get_the_title() : get_the_ID()); ?>"><?php if ( get_the_title() ) the_title(); else the_ID(); ?></a></li></ul></li>
		<?php endwhile; ?>
		</ul>
		<?php echo $after_widget; ?>
<?php
		// Reset the global $the_post as this query will have stomped on it
		wp_reset_postdata();

		endif;

		$cache[$args['widget_id']] = ob_get_flush();
		wp_cache_set('widget_recent_posts', $cache, 'widget');
	}

	function update( $new_instance, $old_instance ) {
		$instance = $old_instance;
		$instance['title'] = strip_tags($new_instance['title']);
		$instance['number'] = (int) $new_instance['number'];
		$this->flush_widget_cache();

		$alloptions = wp_cache_get( 'alloptions', 'options' );
		if ( isset($alloptions['widget_recent_entries']) )
			delete_option('widget_recent_entries');

		return $instance;
	}

	function flush_widget_cache() {
		wp_cache_delete('widget_recent_posts', 'widget');
	}

	function form( $instance ) {
		$title = isset($instance['title']) ? esc_attr($instance['title']) : '';
		$number = isset($instance['number']) ? absint($instance['number']) : 5;
?>
		<p><label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?></label>
		<input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo $title; ?>" /></p>

		<p><label for="<?php echo $this->get_field_id('number'); ?>"><?php _e('Number of posts to show:'); ?></label>
		<input id="<?php echo $this->get_field_id('number'); ?>" name="<?php echo $this->get_field_name('number'); ?>" type="text" value="<?php echo $number; ?>" size="3" /></p>
<?php
	}
}


function WP_Widget_RecentPostInfoInit() {
	//ウィジェットのクラス名を登録
	register_widget('WP_Widget_RecentPostInfo');
}
//widgets_initアクション時にwpNdTwitterInit関数を実行
add_action('widgets_init', 'WP_Widget_RecentPostInfoInit');


?>